Paul there are a few other ways to improve a thin voice.

You could add a bit of compression.

There are plugin warmers. Check out PSP vintage warmer.

You could adjust the eq to make the sound richer.

Reverb and FX will also help.

Sometimes a little panning helps.

One technique worth trying is to duplicate the vocal track and offset it very very slightly rather than recording a separate double track which is vey difficult to achieve as no two vocals will match properly. You will have to experiment to find the optimum balance and time delay between the two tracks.
I might add that sorting and matching double tracking is an engineers nightmare. Most singers can never sing exactly the same twice.

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