no the lucky egg is only found on a chansey, and it doesnt up your exyp by 50%, it doubles it wich is better! i have one!
PS:the chansey is very rare and to make things worse, you only have a 5% chance of haveing it on a chansey! ;_;
You can get it in XD and off Chansey, but most importantly the Lucky egg hold item does not double the EXP as here is an example:
For example the pokemon that you have defeated has given you 2000 EXP
Your own pokemon on a normal basis gets= 100% of the 1000 EXP
EXP gain= 1000
Traded pokemon gets= 150% of the 1000 EXP
EXP gain= 1500
Lucky Egg holding pokemon gets= 150% of the 1000 EXP
EXP gain= 1500
Traded Pokemon, holding Lucky Egg gets= 200% of the 1000 EXP
EXP gain= 2000
As you can see from that example a pokemon that is your own and is holding the lucky egg will get an 50% more EXP then normal, which is the same amount of extra EXP that a traded pokemon gets.
Therefore only if a pokemon that is not your own (different id number) and is holding a Lucky Egg will get double the EXP.
I hope this helps as I have checked this out on quite a few occasions while facing the Elite 4 in the GBA games

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