An item with the same key has already been added

[ProNotion]

Did anything change recently that might cause me to start getting these sorts of errors?

Here are two such examples:

System.ArgumentException: An item with the same key has already been added. Key: nextToken
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Linq.Enumerable.ToDictionary[TSource,TKey,TElement](IEnumerable`1 source, Func`2 keySelector, Func`2 elementSelector, IEqualityComparer`1 comparer)
   at FikaAmazonAPI.AmazonSpApiSDK.Runtime.AWSSignerHelper.ExtractCanonicalQueryString(RestRequest request)
   at FikaAmazonAPI.AmazonSpApiSDK.Runtime.AWSSigV4Signer.CreateCanonicalRequest(RestRequest restRequest, String signedHeaders)
   at FikaAmazonAPI.AmazonSpApiSDK.Runtime.AWSSigV4Signer.Sign(RestRequest request, String host)
   at FikaAmazonAPI.Services.TokenGeneration.SignWithSTSKeysAndSecurityTokenAsync(RestRequest restRequest, String host, AmazonCredential amazonCredential)
   at FikaAmazonAPI.Services.RequestService.ExecuteRequestTry[T](RateLimitType rateLimitType)
   at FikaAmazonAPI.Services.RequestService.ExecuteRequestAsync[T](RateLimitType rateLimitType)
   at FikaAmazonAPI.Services.FbaInventoryService.GetInventorySummariesByNextTokenAsync(String nextToken, ParameterGetInventorySummaries parameterGetInventorySummaries)
   at FikaAmazonAPI.Services.FbaInventoryService.GetInventorySummariesByNextToken(String nextToken, ParameterGetInventorySummaries parameterGetInventorySummaries)
   at FikaAmazonAPI.Services.FbaInventoryService.GetInventorySummariesAsync(ParameterGetInventorySummaries parameter)

System.ArgumentException: An item with the same key has already been added. Key: marketplaceIds
   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](IEnumerable`1 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)
   at FikaAmazonAPI.AmazonSpApiSDK.Runtime.AWSSignerHelper.ExtractCanonicalQueryString(RestRequest request)
   at FikaAmazonAPI.AmazonSpApiSDK.Runtime.AWSSigV4Signer.CreateCanonicalRequest(RestRequest restRequest, String signedHeaders)
   at FikaAmazonAPI.AmazonSpApiSDK.Runtime.AWSSigV4Signer.Sign(RestRequest request, String host)
   at FikaAmazonAPI.Services.TokenGeneration.SignWithSTSKeysAndSecurityTokenAsync(RestRequest restRequest, String host, AmazonCredential amazonCredential)
   at FikaAmazonAPI.Services.RequestService.ExecuteRequestTry[T](RateLimitType rateLimitType)
   at FikaAmazonAPI.Services.RequestService.ExecuteRequestAsync[T](RateLimitType rateLimitType)
   at FikaAmazonAPI.Services.FbaInventoryService.GetInventorySummariesByNextTokenAsync(String nextToken, ParameterGetInventorySummaries parameterGetInventorySummaries)
   at FikaAmazonAPI.Services.FbaInventoryService.GetInventorySummariesByNextToken(String nextToken, ParameterGetInventorySummaries parameterGetInventorySummaries)
   at FikaAmazonAPI.Services.FbaInventoryService.GetInventorySummariesAsync(ParameterGetInventorySummaries parameter)

Both appear to be occurring on calls where the nextToken is passed.

[TechyChap]

I've seen this when 2 threads both access the connection at the same time. #443