making a passphrase by doubling a password and tweaking the end

[Anonyma]

(This is as much about ssh as gpg, but I figure there should be some
passphrase expertise here.)

Suppose my shell password is "SapNilph4" (I just got that from APG),
is it stupid to make a passphrase for an ssh or gpg key by doubling it
and changing the end, for example "SapNilph4SapNilph3"?  Or am I
really wasting potential entropy this way?

thanks