Quote Originally Posted by DIANESDOGS
Can it???
Yes it can, and it is.

Copy-pasted from gemini's link:

Q: Is it possible for one litter to contain puppies from two different male dogs?
A: This is possible. Although each ovule can only be fertilized once, in each heat stage a lot of ovules mature, which is why it is possible for a female dog to become impregnated from several different male dogs if she has copulated with more than one. If you are intending for your female dog to become impregnated, it will be necessary to keep the dog away from other dogs until the heat stage has completely ended.