Google Forms can block repeat submissions, but only if people sign in with a Google account. Here is what each setting does, and what to do when you can't require sign-in.
Respondents now have to sign in to Google, and each account can submit only once.
The catch: people without a Google account can't fill in your form, and some will simply leave. Someone with two Google accounts can still submit twice.
In Settings → Responses → Collect email addresses, choose Verified. Each response is tagged with the respondent's real Google account email, so duplicates are easy to spot later. This also requires sign-in, but it doesn't stop a second submission by itself.
Many duplicates come from people who want to fix a typo and submit again. In Settings → Responses, turn on Allow response editing. People get a link to edit their answer instead of sending a new one.
Event sign-ups, customer orders, school notices to parents, and public surveys often can't require a Google login. In that case you can't block duplicates up front. What you can do:
| Setting | Blocks duplicates? | Requires sign-in? |
|---|---|---|
| Limit to 1 response | Yes, per Google account | Yes |
| Collect verified emails | No, but makes them easy to find | Yes |
| Allow response editing | Reduces them | No |
| Clean up with Duplicate Cleaner | Removes them afterwards | No |