I wish to know if you get a double entry tourist visa do you forfiet the visa if you leave the country and with the double entry visa do you just have to report to immigration office or cross the border for re-entry

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A visa entry is used on entry - an exit ends an entry permitted to stay. If a two entry visa you can still use the second entry for your next visit if the visa itself is still valid.

A new entry is activated on entry to country from another country. You can not do that at an Immigration office.

You do have the option to extend a 60 day tourist visa entry for 30 days.

Edited October 9, 200916 yr by lopburi3

An option could be to obtaining a re-entry permit from immigration before you leave the country to preserve your permission to stay. On re-entering the country you would be stamped in for the remainder of that stay. A re-entry permit does not give you any additional days. Cost 1000 baht single entry, 3800 baht multiple entry.