If it was me that I'd taken the extra dose, I doubt that I'd be worried.
However the
official advice is this:
"Taking extra doses of some medicines can be harmful. In some cases even one extra dose can cause you problems. If you take extra doses of your medicine, you must get medical advice immediately. You may need a test to assess the effect of taking extra doses. This is because the effects of taking too much medicine are very complex so it is very important that you seek medical advice.
Contact your prescriber, pharmacist, specialist clinic or call 111 for advice.
Make sure you take all of your medicine containers with you if you are advised to go to hospital"
http://www.nhs.uk/Medicine-Guides/pages/MedicineAbout.aspx?condition=bacterial%20infections&medicine=amoxicillin%20trihydrate&preparation=amoxicillin%20500mg%20capsules