Are these 2 alerts for your Yahoo mail (one from whatever app you are using for that and one from the GMail app), or is your Yahoo mail being copied to your GMail account? Your post isn't clear on this.
If it's the first, just remove the Yahoo account from one of the email apps.
If it's the second, that would imply that you have your Yahoo account set up to forward a copy of your mail to the GMail account. In this case the solution is to go into your Yahoo account settings and turn the mail forwarding rule off.
Or you could just turn off notifications in the GMail app, but that would still be wasting resources if it's receiving a copy of everything.