The re-melt temperature of toner for the 1st side of the image is differs at one round (204mm) of the pressure roller and at subsequent areas. Therefore, uneven gloss occurs at 204mm within the image leading edge and at other areas of the image.
1st side of image during duplex printing
[A]: Pressure roller for interleaf (high temperatures result in a particularly high level of glossiness)
[B]: High level of glossiness at one round (204mm) of the pressure roller
This is likely to occur if:
Printing a high coverage image in the duplex mode
Using thick paper