Font Book and duplicates. Strange Behaviour?

Font Book is rather hit & miss. If you want, you can enable the System fonts again and disable the MS ones instead, but you'll need to do it manually. Right-click (or ctrl-click) on the font in the list in Font book and choose enable or disable as appropriate.

For future reference, Kurt Lang's article on Font Management in OS X;

http://www.jklstudios.com/misc/osxfonts.html

The behaviour you described is normal for Snow Leopard.

And yes, you do want to remove the Microsoft fonts, because the ones bundled with the OS have a larger complement of glyphs. Quit Font Book, move the </Library/Fonts/Microsoft> folder out of the Fonts folder, then launch Font Book again and enable as needed. I follow the old practice and create a Fonts (Disabled) folder where I put such fonts (just in case I might need them at some point in the future).

For details, read Kurt Lang's paper on Font Management. I'd say it's required reading, and if you have any questions about it you can ask the author -- he's often around.

<http://www.jklstudios.com/misc/osxfonts.html>

Disabling it is usually adequate, but there are times when the whole lot gets reset to defaults. Booting into Safe mode is one. So it's generally better to move the font out of FB's reach or delete it altogether if it won't be needed.

Kurt's article discusses the merits of other font management tools which may be of interest.

S_t_e_p_h_e_n wrote:

why does one have to remove a font

It's easier to move one folder than manually to select and disable 20 or 30 fonts in Font Book. And there are other reasons, too. A lot will become clearer if you read Kurt's paper.