@Aniket Shukla 

you have only 1 transform map and you want to set 2 target fields with same source value?

If yes then system is blocking you?

If yes then why not use onBefore transform script for the 2nd field like this

target.fieldName = source.u_fieldname;

If my response helped please mark it correct and close the thread so that it benefits future readers.

@Aniket Shukla 

the second field seems to be a dot walked field

share screenshots.

@Aniket Shukla 

which are the other field maps?

Did you check any other field map already exist for the same target field?

If my response helped please mark it correct and close the thread so that it benefits future readers.