Fix creating sync subscription for the same method and path

[mthenw]

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[codecov]

Codecov Report

Merging #476 into master will increase coverage by 0.06%.
The diff coverage is 89.28%.

@@            Coverage Diff             @@
##           master     #476      +/-   ##
==========================================
+ Coverage   70.66%   70.73%   +0.06%     
==========================================
  Files          35       35              
  Lines        2175     2180       +5     
==========================================
+ Hits         1537     1542       +5     
  Misses        574      574              
  Partials       64       64

Impacted Files	Coverage Δ
internal/cache/target.go	0% <0%> (ø)	⬆️
libkv/subscription.go	88.75% <100%> (ø)	⬆️
internal/cache/subscription_cache.go	87.67% <95.65%> (+0.9%)	⬆️

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