Can I use "Signs of Evolution" to get two of the same cards that evolve from Eevee if I don't have 3 different ones in my deck?
No. Each one you take out of your deck must be a differently typed evolution of Eevee; you may not retrieve more than one Pokemon of the same type.
Source: Plasma Freeze FAQ (2013-05-16), TPCi Rules Team (2013-05-16)

Relevant cards

Note: All applicable cards may not be listed below.
EX2_63
EX6_56
PL2_59