It is normal to have double teeth and they will be removed at the time of spay or neuter (about 6-7 months old).
As for toothpaste - only use a pet toothpaste. I like the CET dental by Virbac products. We use Vanilla mint flavor but they do have other flavors as well. This is the one we buy -
Amazon.com: C.E.T. Toothpaste - 70 g - Vanilla...
To brush her teeth, use a soft baby toothbrush and let her lick a small amount of the toothpaste off the brush and encourage her with all kinds of praise. Every time she accepts the toothbrush, reward her with praise and treats. work your way up to her allowing you into her mouth and gently brush her teeth and gums. Oh and don't forget to tell her what you are doing. I always tell them what we are doing.