I (accidentally) found that there are duplicated images in ImageNet dataset, as shown below.
| Item | Number of images |
|---|---|
| Entire dataset | 1,281,167 |
| Unique images | 1,275,220 (99.5%) |
| Images that occur 2 times | 5,727 |
| Images that occur 3 times | 107 |
| Images that occur 4 times | 2 |
The duplications happen both across different classes (e.g., n03372029_47612 and n03884397_26733) and within the same class (e.g., n02088632_982 and n02088632_819). This could potentially lead to problems if the code or evaluation assumes no duplication in the dataset.
I could not find public documentation about this issue. I shared the list of duplicated images in this CSV file in case anyone needs it. Each row lists the file names of one duplicated image.