Covering for a coworker once is kind. Covering every week teaches them nothing.

Help has a point where it stops helping. Covering once for somebody in a genuine bind is a kindness and it is what makes a team worth being on; covering every week removes any reason for them to sort the problem out and quietly turns their job into yours. Both of you get worse out of that arrangement. Generosity needs an edge on it somewhere.

Say yes to the emergency and start saying no to the pattern. Name it plainly and kindly the second or third time. Offer to help fix the cause rather than absorbing the effect again.