I inadvertently joined Climate Prediction twice, firstly in 2004 and more recently in 2007. My user name is identical in each case but only the newer membership is 'running'. Is it possible to merge the two users so that my total work done is shown as a single total?

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This is a common question with a simple answer: No
People are identified by their email names.

Your choices are:
Pick the one with the most credit.
Pick the one with the most completed models.
Pick the one that's easiest to use.

Ignore the other one.

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Backups: Here

Thanks for that explanation. My earlier membership (with the largest credit!) was using an email address which no longer exists. So, I guess I'm stuck with the latest version with lower credit, no problem!

You don't need an active email address to use the older account. The servers don't use it.
But you do need to know what your old name and email name were.

The exact procedure to use will depend on what info you have from before. For instance, if it was all on on old computer that you no longer have, then you may not know your original security code, which is in the file: account_climateprediction.net.xml, and can be used instead of the email name to login.