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.

Demo: https://woo.ciphercoin.com/unique/

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.