Hi Nicky, I'm a twin.
Biologically twins are only in the mother's genes (so the scientists say), with the exception of modern day I.V. treatment.
Saying that, the amount of twins I've found in my One Name Study - mmmm quite a few.
On the profiles within the trees/profiles I create, under biography - I always state 'twin'.
So far I've not come across triplets etc...
In answer to your question "Is there a way to see the frequency" - I don't believe there is a way to click on a button etc to see this.
I like the idea/suggestion - something on the profile to indicate multiple births.