I just purchase some new fonts and they are all Open Type font. When I use in Photoshop(7), I notice that everytime I have a double letter (e.g. coffee, pool,apple), one of the letter is upper case and the other is lower case. Is there a way that I can change it so it won't be like that? I ask the creator of the fonts about it but he just tell me that it is a feature of Open Type font. What can I do to have it just in the same case when I use the font?
I think you mean that if you type "coffee and "pool", what you see is "coFfEe" and "pOol". This is certainly not a standard way that OpenType works. It is unlikely to to be an Adobe Photoshop bug, as this problem would have shown up by now in many documents.
Photoshop will apply certain standard OpenType layout features, such as 'liga', which replaces combinations of two or more glyphs wih a single ligature glyph. This may be the case here. What actually happens when the 'liga' (or any other OpenType layout feature) is applied depends how the font is is built.
Note that I am using the phrase "layout feature" to mean something very specifc, not just any property of a font. An OpenType layout feature is defined in the OpenType specification. A layout feature is basically an ordered list of rules, where each rule specifies a glyph sequence to match in a string of text, and specifies what should be done when that sequence is found. The order of the rules is very important, as a text layout program is supposed to stop at each position in a string of text, and then look through all the rules to find the first one that matches. If you put the rules in the wrong order, then you can get effects you don't want, like replacing "ffi" with an f_f ligature followed by "i", rather than with the three-letter ligature f_f_i.
It sounds like the font developer did this on purpose, using OpenType layout features that are on by default in Photoshop. Is this some sort of unusual looking font from some small, cutting-edge type foundry?
If that's what they did, you can turn it off. Open the Character Palette. Use the flyout menu (from the little triangle at the top right of the palette). Go to the "OpenType" submenu, and then turn off "contextual alternates." I'll bet the problem goes away.