Guard against multiple substitution group runs

[tefra]

No description provided.

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #538 (5361381) into master (e83e202) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #538   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           94        94           
  Lines         7685      7689    +4     
  Branches      1327      1328    +1     
=========================================
+ Hits          7685      7689    +4     

Impacted Files	Coverage Δ
xsdata/codegen/handlers/attribute_substitution.py	100.00% <100.00%> (ø)
xsdata/codegen/models.py	100.00% <100.00%> (ø)

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