Don't cache same STIX bundle more than once

[pcoccoli]

Mitigates (but does not resolve) #207

[codecov]

Codecov Report

Merging #225 (5c53787) into develop (fc6e1e1) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #225      +/-   ##
===========================================
+ Coverage    78.83%   78.90%   +0.07%     
===========================================
  Files           38       38              
  Lines         2353     2361       +8     
===========================================
+ Hits          1855     1863       +8     
  Misses         498      498              

Impacted Files	Coverage Δ
src/kestrel_datasource_stixbundle/interface.py	65.65% <100.00%> (+2.21%)	⬆️
src/kestrel/session.py	90.00% <0.00%> (+0.08%)	⬆️

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