This contact form 7 addon trigger error if a field is already submitted. (Prevent contact form 7 duplicate submissions – eg: email already exists )
Just add shortcode then you can enable the unique field for your contact form 7.
How to use?
Upload the zip file to your WordPress website and activate it.
Add the shortcode in contact form 7 tags as follows.
Example : [unique your-email your-phone ]
Only $29 for the unlimited installation license.