It might seem excessive, but it is always a good idea to keep a record of your purchases. It can feel burdensome and over the top when you start and have just a few stamps, but as your collection grows – and it will – the creation of some sort of catalogue is highly recommended.
This doesn’t have to be too elaborate, but a simple listing of what the issues are, perhaps by country or species is helpful, along with their postal value, Stanley Gibbons catalogue number (dealers always quote these), their condition – whether they are mint, used, hinged, unhinged, damaged, creased, etc - as well as the name of the dealer and their date of purchase. Some might choose to indicate the price you paid for them, but values can always change.
If you start doing this at the beginning of collecting it becomes second nature and easy to do. It gives you an accurate sense of the size of your collection and, crucially tells you how many duplicates you have acquired. And yes, you will always find you have pesky duplicates!