fix(tx): Fix duplicate entries in metadata.conflict_with

[msbrogli]

Example of the duplicate entries:

[codecov-commenter]

Codecov Report

Merging #250 (7a39a5f) into dev (a4c651e) will decrease coverage by 0.04%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##              dev     #250      +/-   ##
==========================================
- Coverage   82.35%   82.30%   -0.05%     
==========================================
  Files         148      148              
  Lines       14078    14079       +1     
  Branches     2009     2010       +1     
==========================================
- Hits        11594    11588       -6     
- Misses       2069     2073       +4     
- Partials      415      418       +3     

Impacted Files	Coverage Δ
hathor/transaction/transaction_metadata.py	90.44% <33.33%> (+0.56%)	⬆️
hathor/consensus.py	93.10% <100.00%> (-1.34%)	⬇️
hathor/p2p/node_sync.py	83.04% <0.00%> (-0.50%)	⬇️
hathor/p2p/manager.py	68.90% <0.00%> (+0.42%)	⬆️

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