Large item purchased that we already bought!

WanderingA

Our shower is this weekend and I have been failing miserably at not-checking the status of our registries!  I put the travel system that we want on my registry fully intending to just have it on there so that we could get a discount on it after the shower.  Two days ago I checked the registry and it hadn't been purchased so my husband ordered one on e-bay that was a good deal.  Well today I checked my registry and someone had purchased it!  I talked to my sister and she told me that my mother bought it for us.  I am super grateful and amazed that she would get us such a nice present and I don't want to make it seem like I am ungrateful - should I just not tell her that we already have it at the shower and return it afterwards?