When two relations (R1 and R2) share the same way (W1) and R1 changes, R2 will be inserted a second time.
Cause: W1 gets removed and re-processed when R1 changes, because it might now be redundant (e.g. if R1 now has the same tags as W1). When W1 is processed again, it forces the reinsert of R1 and R2. R2 is then inserted a second time since R2 was not removed in the first step.
When two relations (R1 and R2) share the same way (W1) and R1 changes, R2 will be inserted a second time.
Cause: W1 gets removed and re-processed when R1 changes, because it might now be redundant (e.g. if R1 now has the same tags as W1). When W1 is processed again, it forces the reinsert of R1 and R2. R2 is then inserted a second time since R2 was not removed in the first step.