Made an double plat purchase?

[Mladen]

so i bought a pile of plat and later i noticed i got another 30 plat which i didnt buy.. i check my google wallet and see double purchase, am sure i didnt tap accept and buy twice or something.. did this happen to anyone else? is it a google play app bug?
thanks.
p.s i wont really bother for a refund just wanna know how did it happen, if i bought more plat would be nasty..