The CLAMs were given out last summer to every BTC, DOGE, and LTC address that had a non-dust balance on some date in May. 4.6 CLAM to each and every such address.

If you didn't already claim them, your CLAM will still be sitting there in the CLAM blockchain waiting for you to claim them.

The private key for your free CLAM is the same as for your BTC, DOGE, or LTC address which is why you are able to claim your CLAMs and nobody else is able to.