addWithNoDuplicates

Appends toAddSideEffects to this list of side effects, first dropping from its front as many elements as this list has that also occur (by reference identity, not equality) somewhere in toAddSideEffects -- used when merging a child request's accumulated side effects back into its parent's, to avoid recording the same SideEffect instance twice when toAddSideEffects already carries this list's elements as a prefix.