Why are customers being charged twice on my online store?

I had one customer reach out to me about her bank account being charged twice. I only show the transaction once on my end. The two amounts charged are two different amounts (the correct amount and then one that is $5.00 more). I had another customer reach out and her account showed two charges neither of which was the correct amount and neither of which she or I received confirmation for. I’m just curious as to why this is happening and what I need to do to get them their money back for the extra charges.