Does the prayer become valid if the congregant, out of ignorance of the ruling, makes one tasleem, then follows it with two tasleems after the Imam has made his tasleem?

A single taslima is sufficient. However, it is preferable for the worshiper to say two taslimas, and this is the view of the majority of scholars. Scholars have agreed that the prayer of one who limits himself to a single taslima is permissible. Therefore, your prayer is valid, and praying behind an Imam who limits himself to a single taslima is also valid.