The odds are pretty high actually. It's like the shared birthday conundrum - in a group of 23 or more people it's more likely than not that two people will have a shared birthday. Republican denarii were typically struck by dies numbering in the low hundreds, and there is a decent chance of finding die pairs with either other coins in your collection or in some other collection - such as a given sale catalogue . For any one coin of course the probability is low, a fraction of a one percent, but if your collection is large enough, and the number of coins you are comparing to is high, then those fractions of a percent quickly mount up. In my collection I've examples of coins of different types that share on die, for example two denarii of Julius Caesar , on by Sepullius Macer with standing Venus , the other by Aemilius Buca with seated Venus , same obverse die.But it's still lovely to find, just as it's lovely to find a provenance , or someone who shares your birthdaynot rare , but still uncommon enough that it's a nice occurrence.