Is there a difference between 夂 and 夊?
The difference between the two in writing should be clear from these drawings: http://en.wikipedia.org/wiki/Radical_35 and http://en.wikipedia.org/wiki/Radical_34 and the stroke animations from http://en.wiktionary.org/wiki/%E5%A4%8A
and http://en.wiktionary.org/wiki/%E5%A4%82

夂 is normally used at the top (for example 条條修脩务務夐) or the left (for example 处) of a character, and 夊 at the bottom (for example 愛夏复) or the right (for example 致, see http://zhongwen.com/d/173/d80.htm).

Originally both characters were quite different (see http://en.wiktionary.org/wiki/%E5%A4%8A
and http://en.wiktionary.org/wiki/%E5%A4%82), but nowadays there are typically both written as 夂 by most people and most fonts don't make the distinction anymore.