Use the "Resolve Duplicates" in the Edit menu to figure out if they're really dupes. In my "All Fonts" collection I have Arial, Arial Black, Arial Hebrew, Arial Narrow, Arial Rounded MT Bold and Arial Unicode MS. I don't have two plain "Arial" sets.
However, that's in FontBook. If I look in the Finder, I have the following in the Fonts folder:
Arial Black.ttf
Arial Bold Italic.ttf
Arial Bold.ttf
Arial Italic.ttf
Arial Narrow Bold Italic.ttf
Arial Narrow Bold.ttf
Arial Narrow Italic.ttf
Arial Narrow.ttf
Arial Rounded Bold.ttf
Arial Unicode.ttf
Arial.ttf
ArialHB.ttf
ArialHBBold.ttf
And when I asked FontBook to reveal the Arial (plain) in the Finder, I got this - IN a Microsoft folder inside the Fonts folder:
Arial Rounded Bold
Arial Narrow
Arial Black
Arial
(I think these are Postscript fonts - the type is listed as "Font Suitcase")
"Resolve Duplicates" doesn't always distinguish between font types it would seem - because if I look at Arial Bold Italic, for instance, which seems to have a duplicate, I find one is a .ttf font in the Library/Fonts folder and the other is part of the Arial Font Suitcase in the MS folder. I'm not going to remove either - they're not actually duplicates.
Anyway, Arial is not a system font, so if you were to remove it, the system would still work. However, it's a pretty basic font and some applications will not work when you remove their fonts (I suspect that since MS Office installed the Arial fonts, it would be best to leave them there!).
Bottom line - your Arial fonts probably aren't duplicates, but you can check this by "reveal in Finder" - it will show you the location and the
font type.
System fonts:
http://en.wikipedia.org/wiki/List_of_typef...d_with_Mac_OS_X.ttf fonts are TrueType fonts. There are a number of font types supported in OS X:
OpenType (both Mac and Windows formats) .otf
PostScript Type 1 (Mac only)
PostScript Multiple Master (Mac only) MM
TrueType (both Mac and Windows formats) .ttf
dFont (data fork TrueType) (Mac only)
Some applications support some font formats and not others. And example is Open Office - which supports .ttf, but not .otf.
Anyway, unless you have OBVIOUS duplicates, it's best to leave things as they are unless a font is actually corrupted, in which case you should endeavor to replace it from the original software install. Also, when resolving duplicates, don't take anything out of the system folder!