This issue can happen when using amavis.
In case you have a double email signature you can edit your postfix config.
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However, mostly double dkim is NOT an issue, you need to be sure that your dkim signature is added on the DNS servers (if you are using an external DNS).
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File: /etc/postfix/master.cf
smtp inet n - n - - smtpd -o content_filter=smtp-amavis:127.0.0.1:10024 -o receive_override_options=no_address_mappings
After no_address_mappings add no_milters , example:
smtp inet n - n - - smtpd -o content_filter=smtp-amavis:127.0.0.1:10024 -o receive_override_options=no_address_mappings,no_milters
After changes restart postfix:
service postfix restart