System.ArgumentException when accessing miner API
### Description

An API error occures when accessing /api/pools/{pool}/miners/{address}. 

### Steps to Reproduce

This happened on a completely new set up pool for Ethereum Classic. Three miners are mining on the pool. When trying to access the api page for 2 of the three miners, the following error appears.

### Expected behavior

JSON response with miner summary.

### Actual behavior

500 Error and an error stack that is shown below.

### Reproduces how often

Reproduces every time after it initially appears (not completely sure why some addresses work and others not).

### Version

Running clone of master branch to commit 849cc0d

### Additional Information

Copied from api.log:

[2019-05-31 12:15:46.1680] [I] [WebHost] Request starting HTTP/1.0 GET http://api/api/pools/etc1/miners/0x1B79f9689374915c768aCB95a4C3A4C9f9bd4d3A   
[2019-05-31 12:15:46.1680] [I] [EndpointMiddleware] Executing endpoint 'Miningcore.Api.Controllers.PoolApiController.GetMinerInfoAsync (Miningcore)' 
[2019-05-31 12:15:46.1745] [I] [EndpointMiddleware] Executed endpoint 'Miningcore.Api.Controllers.PoolApiController.GetMinerInfoAsync (Miningcore)' 
[2019-05-31 12:15:46.1745] [E] [Kestrel] Connection id "0HLN5NL54J37H", Request id "0HLN5NL54J37H:00000001": An unhandled exception was thrown by the application. System.ArgumentException: An item with the same key has already been added. Key: VOLI-R0-M1211
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](TSource[] source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at Miningcore.Persistence.Postgres.Repositories.StatsRepository.GetMinerStatsAsync(IDbConnection con, IDbTransaction tx, String poolId, String miner) in /var/miningcore/src/Miningcore/Persistence/Postgres/Repositories/StatsRepository.cs:line 219
   at Miningcore.Extensions.ConnectionFactoryExtensions.RunTx[T](IConnectionFactory factory, Func`3 func, Boolean autoCommit, IsolationLevel isolation)
   at Miningcore.Api.Controllers.PoolApiController.GetMinerInfoAsync(String poolId, String address, SampleRange perfMode) in /var/miningcore/src/Miningcore/Api/Controllers/PoolApiController.cs:line 281
   at lambda_method(Closure , Object )
   at Microsoft.Extensions.Internal.ObjectMethodExecutorAwaitable.Awaiter.GetResult()
   at Microsoft.AspNetCore.Mvc.Internal.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeActionMethodAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeNextActionFilterAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ActionExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResourceFilter()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeFilterPipelineAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeAsync()
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Builder.Extensions.MapMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Builder.Extensions.MapMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.ResponseCompression.ResponseCompressionMiddleware.Invoke(HttpContext context)
   at Miningcore.Api.Middlewares.IPAccessWhitelistMiddleware.Invoke(HttpContext context) in /var/miningcore/src/Miningcore/Api/Middlewares/IPAccessWhitelistMiddleware.cs:line 41
   at Miningcore.Api.Middlewares.IPAccessWhitelistMiddleware.Invoke(HttpContext context) in /var/miningcore/src/Miningcore/Api/Middlewares/IPAccessWhitelistMiddleware.cs:line 41
   at Miningcore.Api.Middlewares.ApiExceptionHandlingMiddleware.InvokeAsync(HttpContext context) in /var/miningcore/src/Miningcore/Api/Middlewares/ApiExceptionHandlingMiddleware.cs:line 20
   at AspNetCoreRateLimit.RateLimitMiddleware`1.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication`1 application)    at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](TSource[] source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at Miningcore.Persistence.Postgres.Repositories.StatsRepository.GetMinerStatsAsync(IDbConnection con, IDbTransaction tx, String poolId, String miner) in /var/miningcore/src/Miningcore/Persistence/Postgres/Repositories/StatsRepository.cs:line 219
   at Miningcore.Extensions.ConnectionFactoryExtensions.RunTx[T](IConnectionFactory factory, Func`3 func, Boolean autoCommit, IsolationLevel isolation)
   at Miningcore.Api.Controllers.PoolApiController.GetMinerInfoAsync(String poolId, String address, SampleRange perfMode) in /var/miningcore/src/Miningcore/Api/Controllers/PoolApiController.cs:line 281
   at lambda_method(Closure , Object )
   at Microsoft.Extensions.Internal.ObjectMethodExecutorAwaitable.Awaiter.GetResult()
   at Microsoft.AspNetCore.Mvc.Internal.ActionMethodExecutor.AwaitableObjectResultExecutor.Execute(IActionResultTypeMapper mapper, ObjectMethodExecutor executor, Object controller, Object[] arguments)
   at System.Threading.Tasks.ValueTask`1.get_Result()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeActionMethodAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeNextActionFilterAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Rethrow(ActionExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.InvokeInnerFilterAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResourceFilter()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeFilterPipelineAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeAsync()
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Builder.Extensions.MapMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Builder.Extensions.MapMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.ResponseCompression.ResponseCompressionMiddleware.Invoke(HttpContext context)
   at Miningcore.Api.Middlewares.IPAccessWhitelistMiddleware.Invoke(HttpContext context) in /var/miningcore/src/Miningcore/Api/Middlewares/IPAccessWhitelistMiddleware.cs:line 41
   at Miningcore.Api.Middlewares.IPAccessWhitelistMiddleware.Invoke(HttpContext context) in /var/miningcore/src/Miningcore/Api/Middlewares/IPAccessWhitelistMiddleware.cs:line 41
   at Miningcore.Api.Middlewares.ApiExceptionHandlingMiddleware.InvokeAsync(HttpContext context) in /var/miningcore/src/Miningcore/Api/Middlewares/ApiExceptionHandlingMiddleware.cs:line 20
   at AspNetCoreRateLimit.RateLimitMiddleware`1.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication`1 application)
[2019-05-31 12:15:46.1848] [I] [WebHost] Request finished in 16.7774ms 500  
