Hello, I assume you are using the dialogue that asks if you want to duplicate the attachment for each skin when making a new skin placeholder?
Skins - Spine User Guide: New skin placeholder
Simply do not check that option, and instead select the attachment inside the skin placeholder, duplicate it, move it outside the skin placeholder, select the actual skin you want it into, and move it back into the skin placeholder. Then change the image path if it needs to feature a different image.
If a skin should be an exact recolor of another, it may also be quicker to select the skin you want to copy from the skins node, duplicate it, assign the name of the folder where teh new images are located, then check "Rename attachments" to have the above procedure automatically performed on all the attachments belonging to the skin. Skins - Spine User Guide: Skin
