I've been collecting ancient coins for three years now and I've gotten used to finding duplicates while browsing online auctions. However, I don't feel at ease yet explaining it. Quite a few of them will be fakes, but you're also assured (specially by sellers) that such coincidences are perfectly possible. The way I see it there are three options for each case: A) All genuine centuries-old coins from a contemporary die. B) All fakes from a modern die. C) At least one fake from a die obtained from a genuine coin, which might or might not be among the samples. Assuming every collector has faced this dilemma, what do you usually focus on to get to your conclusion? Here's two examples to aid the discussion: https://www.acsearch.info/search.html?id=435936 https://www.acsearch.info/search.html?id=2162507 https://www.acsearch.info/search.html?id=2529703 https://www.acsearch.info/search.html?id=1845252 https://www.acsearch.info/search.html?id=2213100 https://www.acsearch.info/search.html?id=68131 (By the way, I've checked and none of them are featured at Forgery Network of FAC's database, so we're on our own.) Frankly, looking at each one separately I wouldn't suspect much (though the ones in the second set look odd), but common sense dictates than when dealing with mass produced objects such as these duplicates should be rare - and here we have triplicates. Further, differences in wear are slight at best. Also, notice in the first set a couple of them have that "cabinet toning" dealers like to point out, so they certainly are somewhat old (unless "cabinet toning" can be faked, which I've never heard about).
Hold your horses. I'm sure each die could produce a few thousand coins before it was useless. Many of the coins burried might have come recently from the mint prior to burial, and I'm sure it would not be unusual for some of the hoards people burried to contain Manu coins with die matches. Die matches, although not common, are not exactly unheard of.
Die matches are perfectly normal, and actually very useful for helping to uncover forgeries if you can confirm the match against a known authentic coin such as in the British Museum or elsewhere. Coin matches, on the other hand, are problematic as it concerns authenticity because it's pretty unlikely that 2 hand made ancient coins are exactly the same. Of course, you can have die matches with fake dies where multiple forgeries exist so the key is to study the issue and find examples of likely authentic examples to compare it. Whether your examples are authentic, I have no idea because I'm not familiar with the types. The 2nd group does not all share the same reverse die IMO.
Die duplicates are not all that uncommon if you are studying an issue. There are even coin types that are only known from one die set so every coin of the type you see will be a duplicate. It is certainly much more unusual to find a die pair from a common issue like a late Roman bronze or Athenian tetradrachm. Being a die duplicate means nothing when considering if a coin might be fake but finding a die duplicate that is off center or damaged in exactly the same way certainly is a VERY bad sign. I have a page showing a few examples: http://www.forumancientcoins.com/dougsmith/dielink.html On that page I have a little exercise for those new to the concept. In the image are coins that share obverse, reverse or both dies. Click on image to enlarge. Answers are on the page at the very bottom.
Note the difference between die matches and whole coin matches. Die matched coins are struck from the same dies, so all devices and lettering will precisely match (except for differences attributable to die wear), but flan shape, strike, wear, surface conditions, nicks, patinas, etc. will differ. Whole coin matches are where everything about two coins is identical, including devices, flan shape, strike, surface conditions etc. Die matches are frequently found between genuine ancient coins. Total coin matches between ancient coins are only found where one or both of the matched ancient coins is fake. Of course, I'm not saying that die matched coins are always genuine. Certainly, modern fake dies are used to strike coins.
Die matches are found on authentic coins, but two coins shouldn't be identical in terms of striking flan orientation ect. I say shouldn't because there can always be an exception though extremely remote.
Thanks for your answers, I'm glad I asked. Indeed, I wasn't paying due attention to the difference between die match and coin match. Though I've seen a few cases where even that distinction can be hard to make. I'll try to post an example of this later. Regarding the first group of coins I did find a duplicate in the British Museum collection. Curiously that coin was added to their collection in 1981, while one of the auctions gives a provenance up to 1982, so its possible they were all part of the same early-80s hoard. I didn't find a duplicate for the second group obverse anywhere apart from those three, and I still think they look odd, but the fact it features at least two different reverses is a point in its favor. In short, I guess the best we can do is doing as much research as possible on the type we want to collect. Got it.
