The Palette Manager can compress the palette to remove duplicate entries from a paletted image.
Duplicate entries are two or more indexed entries that share the same RGB values. The unused entries are changed to black (0,0,0) and the color table is resorted according to the current sorting scheme. Transparent colors are not compressed.
The Compress button is not available for grayscale images.
As with most changes in the Palette Manager dialog box, the changes you make to the palette are not permanent until you click the Apply button.