If your photo is rejected because it is a 'duplicate', this means that you have a similar shot already uploaded in the P-M database.
To qualify as a 'duplicate', the two pics must (i) be of the same aircraft type with the same registration, (ii) be taken at a similar angle, (iii) have little or nothing to choose between them in terms of their composition and/or feature the same background. For the avoidance of doubt, these criteria apply even if the photos in question were taken on different dates. Also, please note that the angle between individual uploads must differ by at least 45 degrees.
For pictures taken at different airports, one similar photo will be accepted.