We need to install two plugins. Install contact form 7 database add-on https://wordpress.org/plugins/contact-form-cfdb7/ and duplicate control add-on (contact form 7 prevent duplicate submissions)
Demo: https://woo.ciphercoin.com/unique/
Contact form 7 prevent duplicate emails in demo. I added an email as a unique field. So you can see error message email already exists if submitted the same email again. This plugin is helping to limit submissions.
How to limit duplicate submissions?
Just add a unique shortcode after submitting a button shortcode. Example: [unique email, your-phone]
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