Possibly unnecessary duplicate loading of all framework libs

[kindermannhubert]

Feature Description

At the start, we load all reference and implementation references. Then when load library targeting the same framework via #load "xxx.dll" all framework references (ref and impl) are loaded again (if dll target same framework) in AssemblyReferenceService.CreateDefaultReferences. Resulting MetadataReferences are then merged (in AssemblyReferenceService.LoadSharedFrameworkConfiguration) with already loaded ones. We should only load references not already loaded.