C front-end: refuse duplicate declaration of local variable

[tautschnig]

Just like GCC, Clang, or Visual Studio we should reject duplicate declarations of local variables in the same scope.

Fixes: #8129

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (79fafbe) 79.69% compared to head (e466dbe) 79.69%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8160   +/-   ##
========================================
  Coverage    79.69%   79.69%           
========================================
  Files         1680     1680           
  Lines       195167   195174    +7     
========================================
+ Hits        155544   155551    +7     
  Misses       39623    39623           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.