I have two Gmail accounts: firstname.lastname@example.org and email@example.com (second one does not have a dot). When I want to sign in to second one, the Gmail signs me into first one. How to stop that please?
Basically both email addresses are same because Dots don't matter in Gmail addresses.
If someone accidentally adds dots to your address when emailing you, you'll still get that email. For example, if your email is firstname.lastname@example.org, you own all dotted versions of your address:
Note: If you use Gmail through work, school, or other organization (like yourdomain.com or yourschool.edu), dots do change your address. To change the dots in your username, contact your admin.