preventing duplicate entries in a table from form submissions

we have a lead capture form that feeds into a tinytable. problem is people sometimes submit the form multiple times (double click, page refresh, genuinely submitting twice etc). we end up with duplicate rows in the table is there a way to check for duplicates before inserting? like "if a row with this email already exists, update it instead of creating a new one" we tried cleaning up duplicates manually but with 50+ submissions a day its a losing battle