Well, there are a number of ways to look at reincarnation:
Same soul, human only
This is the idea that humans reincarnate only as humans (and some don't worry
about if other creatures do or not) and sometimes only as a single gender.
This is the idea that souls stay 'mostly' discrete, but particularly strong
lives (like garlic in soup) flavor other lives and memories. This is one
reason why so many people recall having been particularly prominent lives.
This is the idea, that like compost or rain, all that lives is discreet while
it lives, and then just combines (like a good mulch breaks down, or a raindrop
falls to the sea) and eventually other 'drops' or 'leaves' form. The odds of
the exact same soul forming again are slim.
Now take all these ideas, realize that despite appearances, they are not mutually
exclusive, and let that simmer a while in the back of your brain.