Hank here from Shopify.
That is certainly odd, are they both coming from the one store? Are they appearing within the Settings > Billing section?
Or is there a chance that you have more than one store active? If unsure, you can try this link and it will detail any store that is linked to your email(s).
Are the amounts the same each month and do they both state they are Shopify charges?
Let me know, and I will certainly assist you with getting to the bottom of this.
All the best,