From 40e616a (T274881), Special:PageTranslation?do=mark warns when there is several translation variables (tvar) with the same name, but with different contents.

Since:

• the translation admin can miss that warning when there is many units in the page;
• I don’t see any usecase where we may want a tvar to override another one;

I propose to make this warning to prevent the page to be marked for translation until the issue has been fixed (name changed, or content made identical).