When you say you are using your own form, are you referring to a Google Form, or a custom form in Glide? It’s easy enough to detect and prevent duplicates with Glide custom forms - check that example I linked to.
In terms of creating relations based on dates, the thing you need to understand is that a Glide date column will always be saved as a date and time internally, regardless of what its displayed as. So if you create a relation on raw dates you are likely to get unexpected results.
What you need to do is convert it to a “fixed” value. One option is to use an if-then-else column that returns a true/false result by comparing the date column to some other known value. Do that in both tables, and then create a relation between the two “boolean” columns.
Another option is to use a combination of math and templates columns to “lock” the date to a fixed format excluding the time, eg “yyyy-mm-dd”, and then use the template columns to create the relationship.