PVS-Studio warning V501 for Firebird

int Parser::parseAux()
{
  ....
  if (yyps->errflag != yyps->errflag) goto yyerrlab;
  ....
}

This suspicious code was found in Firebird project by PVS-Studio static code analyzer.
Warning message is:
V501 There are identical sub-expressions to the left and to the right of the '!=' operator: yyps->errflag != yyps->errflag parse.cpp 23523

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