Batch rename: skipping already renamed files

so in one other recent question i needed to find a solution to bulk renaming files by adding .ab into the filename. I succeeded in doing that, but now i face a different problem. Every time i run the batch file, it appends .ab to all the files even those, already renamed. I tried fixing the problem in the following way, which doesn't work.

rem @echo off

FOR /R "C:\Users\" %%G in (*.txt) DO (
  %%G|findstr /i /L ".ab">nul
  IF errorlevel 1 (
    REN "%%G" "%%~nG.ab.txt"
  ) ELSE (
    skip %%G
  )
)

pause

Essentially i need to check if the file name already contains ".ab" in its name and then either skip or add .ab depending on the result. I would appreciate any help.