I am no expert but there may be a few problems with your code (cannot see the indent).
You are always changing position.x never y (might be normal for a platform).
You randomize once in the beginning. that could be the problem.
I would try to make a Global method for randomization :
func roll_dice(start, finish):
var rng = RandomNumberGenerator.new()
return rng.randi_range(start, finish)
And call it everytime you need a random number. This way you should not have the same number:
Coins1.globalposition.x = Global.roll_dice(-3500,6000)
Coins1.globalposition.x = Global.roll_dice(-1000,650)
Yet, you could also change the range according to previous coinspawn (to avoid having them too close, or too random), maybe by having a check like :
if a coin position is closer than 200, reroll dices.
Hope this helps.