it’s not a bug , it’s their number 6 skill.
EDIT: wiki link http://wiki.guildwars2.com/wiki/Clone_%28Snowball_Mayhem%29
(edited by ekkue.3754)
Each profession’s skill #6 is unique. As ekkue pointed out the mesmer’s is to create a clone of itself. http://wiki.guildwars2.com/wiki/Snowball_Mayhem#Profession-specific_skills
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