The Athlon MP is basically an Athlon XP that has been tested/validated to run in a dual-CPU configuration - it's targeted for Dual-CPU workstations. Furthermore, unlike their Athlon XP-counterparts, ALL Athlon MPs have a 133/266MHz FSB.
The Athlon XP is targeted at the Desktop-segment and will NOT run in a dual-CPU configuration(atleast it won't without physical modification of the CPU).
The Duron is targeted at the Value-segment. This means it sells at a very low price and does not perform as good as an Athlon XP or MP. Also, Durons have only 64KB L2-cache(an Athlon XP/MP has 256 or 512KB) and are running a 100/200 or 133/266 MHz FSB.
Btw, the Duron is more like an offspring of the Athlon(XP) family than the other way around
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