The issue is obviously not the toy anymore. We are constantly refereeing small fights and asking 'who had it first'. Parents of two young kids, what reduced the daily competition and what advice sounded good but did nothing?
With grandparents, specific rules worked better than a long parenting philosophy. We picked the two boundaries that mattered most and let smaller things go. My spouse had a different view initially, so we tried it for a few weeks before deciding. We are still not perfect at this, but the arguments are much shorter now. We only reached that conclusion after trying the other approach first.
A small amount of one-to-one time reduced sibling competition more than buying duplicate toys ever did in our house. Discussing it away from the children reduced the back-and-forth. Your child's response may be different, but this was our experience.