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Basically that's the question - I wonder if set_transient() overwrites/updates a transient option with the same key?

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up vote 4 down vote accepted

Yes, key (prefixed with string identifying it as transient) is used as option name when value is inserted in database.

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Yes. As of 4.3.1, handled in wp-includes/option.php using delete_option() + add_option() or update_option(). If using an external object cache, uses wp_cache_set(), defined by memcached as "The value is set whether or not this key already exists in memcached." – here Sep 22 '15 at 19:05

Maybe, but in my case set_transient() has failed when re-used without delete_transient() first. For some time I thought the problem was that the array I was saving as a transient was too large but that wasn't it. I recommend that you delete the transient first if you're having a problem.

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