Dogs give birth to multiple puppies at a time, so technically every puppy in a litter is a twin.  Identical, or monozygotic twins, come from the same egg, and therefore share a placenta and, often, an amniotic sac. 

How to Determine Identical Puppy Twins:

• If you think your pups are really twins, you can take them to get bloodwork done. It won’t take very long at all and after the tests are finished, you will know for sure.
• When the mother dog is giving birth, try to count the placentas. If there is one less placenta than actual puppies, there is a good chance you have some identical twins on your hands.
• When you look at the two pups side by side, they may look incredibly similar. A lot of littermates look similar, but these two will be almost completely identical. As the pups grow, many of their markings will show up in the same places.

Can dogs tell the difference between twins?
Also a study found that trained scent dogs could tell identical twins apart. Yes, a dog can smell the difference between identical twins. Identical twins are not completely the same, and they also have different smells. 

“Jake was out like a light. I was inches away from his face and he didn’t even know it”.  LOL