It is hard to think of anything more frustrating in coin collecting than someone calling your expensive, pride of the collection coin a fake. It's not just the financial loss. You have been betrayed by a seller you trusted. You have been duped. Your pride is at stake. They must be wrong. You have bought it from a respectable dealer or auction house.
In some cases, the coin in question is an obvious fake. You bought it out of ignorance. My advice: smile and bear it. Don't sell it but keep it around to keep you modest and remind you what the cost of ignorance is.
In some cases, there are reasons to doubt the authenticity of the coin, but it looks good. My advice: hear out the people who claim it is a fake. Take their arguments on board. Don't say "I don't think it's a fake". Your judgement is coloured by the fact that you bought it. If you can test the arguments (it's too heavy, the edge is wrong, the piece is cast etc.) do so. If you can't (it looks wrong, the letters look clumsy, funny weak areas) get the advice of a third party. And a fourth if you can...
Most claims of a coin being a fake can be solved quickly. Fortunately, fakers still make a lot of mistakes. However, there is a category where the experts will continue to differ of opinion. This is a frustrating situation. You would like to believe your coin is genuine, yet, whatever you do, doubt will persist. My advice: sell the coin and buy another specimen. You will gain peace of mind.
Since the above is so painful and frustrating, it should be handled by all parties involved with all the diplomacy and tact they can muster. In particular, slight doubts, severe doubts and certainties should be clearly separated. However, ignoring fakes is definitely not the answer. World of Coins is not a site where members cheer each other on so that everyone can feel good. Its mission includes learning and education. It is quoted by third parties, so that ignoring a fake confers respectability on it.
What to do when you are convinced that your coin is in fact a fake? If you have bought it recently, try to get your money back. If not, label it carefully and very clearly as a fake. If you must sell it, sell it as a fake (some jurisdictions may demand that the item itself is marked as a copy.) If you decide that it fits in your collection, FINE! Contemporary counterfeits are very interesting. Modern counterfeits are reference material to help others avoid the trap you fell into.
And what about your pride? Well, consider that you are neither the first, nor the only one who ever bought a fake. I have two expensive pieces in my collection that were determined to be fakes on this site. I can't imagine a serious, advanced collector who has never bought a fake. Let the object of your anger not be the people who claim it's a fake, but the people whose greed has sunk so low that they will produce or knowingly sell fakes.