The dot (.) in a Gmail address has to be included if yours has one. To Groups.io, joe.smith and joesmith are two different people.
I Am On Gmail And I Don't Like Getting Copies Of My Messages
To turn off duplicate emails, log into your account. Then look for the dropdown in the top right corner of the page, which will be either your name or email address. Click that and then click Account. From your Account page, click the Preferences sidebar tab. Look for the 'I always want copies of my own messages' checkbox and uncheck that. Then click the Update Preferences button.
See also Help Center > Members Manual > Understanding Groups.io accounts > Setting account preferences and viewing account information.
I Get a Spoofing Warning On My Emails
If Gmail is hosting your email, but under a domain other than @gmail.com, send an email to email@example.com telling them that you are seeing spoofing warnings, and they will set your emails to rewrite the From line, which will fix the problem.