Hi Roel,
There is actually a pretty cool way to check for duplicates in monday.com.
Create a Connect Boards column and connect the board to itself. Then add the following automation:
And configure it in the following way:
This will create a connection when a duplicate email is detected. You can use the “When Column changes” custom automation trigger to automate further actions, or create filtered views based on the connection column.
You might also want to use the item creation trigger to check for duplicates (this covers all bases):
Hope that helps! Please feel free to reach out if you need help configuring this or how to tie it into further automations.