Transactions authorise twice? Mint card

minuettoallegretto

Several times now - most recently Monday AND Tuesday - payments I have made on my Mint card have been authorised twice by the retailer. I know that the authorisation only lasts a week, and drops off the system after that, but in the meantime the money is not available to spend. For example, £100 available, spend £40 but double authorised, leaves £20 available until a week later when it's back up to £60 available.
My questions are:
1. Why is this happening and how can I check it in the shop?
2. Am I the only person this is happening to?
Any comments/advice/help please!