Thank you For guidance.. I could resolve the error!
For others who facing error and might get helped from my explanation of solution:
where-ever you have used form range and in your form(edit mode) the range(example SHEET 1 : ColA: ) is not getting listed or has disappeared.
STEP 1 : go to your google sheet from where you are populating (my example SHEET 1)
STEP 2: Go to your col/range of range( COLA in my example)
STEP 3: Select the range data and paste in your laptop excel
STEP 4: Sort it
STEP 5: Either eyeball the duplicate or write formular '=A1=A2' in next col
STEP 6: copy the formula to end of your data
STEP 7: all the cells where you see TRUE as value ... these are your duplicate enteries... you have remove/rename these enteries in ur google sheet to handle the form range issue problem.