Grandparent question?

So we have a 15 month old that my parents adore. They ask for her to stay at their house all of the time. Here's my issue and question: they ask us to buy things in duplicate to keep at their house like highchairs, car seats, play pens. When I bring things over in the baby bag they basically scavenge everything out of the bag diapers, wipes and everything else and then ask for me to buy more which they keep there. My daughter has not outgrown the car seat we did give them and personally--I feel like if they want to have her over (keep in mind I am not asking them to babysit)they can either use ours or buy one for themselves. My mom got very upset. What do you think? It isn't that they are poor or on a fixed income. Am I wrong?