What a little Googling will get you, & I quote:

...hoisin-based sauces can be used in place of plum sauce. In fact, plum sauce gets its nickname, “duck sauce” from the fact that westerners initially confused the two, believing plum sauce was served with Peking Duck.

This enlightening bit o' info turns up on Google's second page of results, while a recipe from captures the #1 spot; the recipe source is credited as but my browser couldn't find that, alas!

Anyway, I had kinda suspected as much: that duck sauce is really plum sauce disguised in little one-serving plastic packets. Wannabe ketchup.

Coincidentally, right now we are deluged with raining plums from the tree in the neighbors' backyard. I'd been thinking chutney, but now... hey....