Anybody who does not think that MIL's can do this just out of selfishness, spite, and competition, well, they do not know my MIL!!!
DS is an only child. The one I dreamed of and planned for many many years... Things like Christmas morning.
Guess who had to get him is first bike???
He has well outgrown that one now, so this year he is getting a bigger better bike... DH was stupid enough to mention the word 'bike' when his parents asked about Christmas ideas.
You can bet that I demanded... yes, demanded, that he go back and tell his parents to NOT get DS a bike. This time, as the mom, I am going to be the one to see my sons eyes lite up when he sees that bike from Santa under OUR tree!!!
Sometimes, things like this really do matter. And, believe me, if the DH is on the parents side, that is NOT good.
If inlaws want to spend all kinds of money, then that is fine... But, it is very obvious that when Grandma and Grandpa's name is on that gift, instead of Santa Claus... well, the selfish motives are very obvious.
If Grandma and Grandpa are truly only interested in seeing that the kids get what they want, then, hey, let them send me a check. Otherwise, they should clear the gift ideas with the parents first, and get the specific brands, model numbers, colors, etc.
To the OP, I would bet good money that the telescope that your MIL is sending is nothing as nice as the one you are buying... I would simply let the kids open their Santa gifts first. Then, later, the gifts from Grandma and Grandpa. Obviously, grandma doesn't care that your DD will be opening a duplicate, and inferior gift. If she did care, sorry, but she should have cleared it with you first.