First a list of copied skills (please post others you ran into, I'll add them to this list):
Warrior:
Penetrating Blow = Penetrating Chop, 5 adrenaline
Griffon's Sweep = Leviathan's Sweep, 5 energy, 5 recharge
Galrath Slash = Silverwing Slash, 8 adrenaline
Desperation Blow = Drunken Blow, 5 energy, 7 recharge
Ranger:
Tiger's Fury = Bestial Fury 10 energy, 10 recharge
Dodge = Zojun's Haste 5 energy, 30 recharge
Point Blank Shot = Zojun's Shot, 10 energy, 3 recharge
Penetrating Shot = Sundering Attack, 10 energy, 3 recharge
Monk:
Divine Healing = Heaven's Delight, 10 energy, 2 cast, 30 recharge
Heal Other = Jamei's Gaze, 10 energy, 0.75 cast, 3 recharge
Heal Area = Karei's Healing Circle, 10 energy, 1 cast, 5 recharge
Symbol of Wrath = Kirin's Wrath, 5 energy, 2cast, 30 recharge
Holy Strike = Stonesoul Strike, 5 energy, 0.75 cast, 8recharge
Necromancer:
Shadow Strike = Lifebane Strike, 10 energy, 2 cast, 8 recharge
Vampiric Touch = Vampiric Bite, 15 energy, 0.75 activate, 2 recharge
Touch of Agony = Wallow's Bite, 5 energy, 0.75 activate, 3 recharge
Desecrate Enchantments = Defile Enchantments, 10 energy, 2 cast, 15 recharge
Spinal Shivers = Shivers of Dread, 10 energy, 2 cast, 15 recharge
Mesmer:
Arcane Thievery = Arcane Larcency, 10 energy, 1 cast, 10 recharge
Symphatetic Vissages = Ancestor's Vissages, 10 energy, 1 cast, 3 recharge
Ethereal Burden = Kitah's Burden, 15 energy, 3 cast, 45 recharge
Inspired Enchantment = Revealed Enchantment, 10 energy, 1 cast, 0 recharge
Inspired Hex = Revealed Hex, 5 energy, 1 cast, 0 recharge
Elementalist:
Whirlwind = Teinai's Wind, 10 energy, 0.75 cast, 8 recharge
Earthquake = Dragon's Stomp, 25 energy, 3 cast, 15 recharge
Crystal Wave = Teinai's Crystals, 15 energy, 0.75 cast, 20 recharge
Ice Prison = Teinai's Prison 10 energy, 2 cast, 30 recharge
Now onto possible unbalanced results from being able to use both these skills whithout echoing or renewing (so no mesmer + heavy e-duty or elite; or ele + elite needed).
Warrior Skills:
Penetrating blow and chop are 5 adrenaline skills, in this low adrenaline cost rangeit's wise to stick to 2-3 adrenal skills max, as the netto overall adrenaline gain starts dropping when spamming adrenal skills around as each other skill loses one strike as well so this doesnt provide any significant problems.
Griffon's Sweep is a skill that is not sued often due to energy requirement, strength investments and the effect only occuring when the skill would be evaded. I don't really see any problems with allowing this twice on the same bar.
Galrath slash two times followed by a final thrust might be interesting in adrenal spikes, providing ~220 damage in three seconds. For PvP purposes the space in a warriors bar is allways highly limited so if people really want to use these three skills I think the skill slot lost is a fair price to pay for it.
Desperation blow, don't even feel like saying anything about it, if people want to use two of these be my guest

Ranger Skills:
Tiger's Fury hads a duration of ~10 seconds in most builds using it (e.g. bunny thumpers, iway wars) and it doesnt add anything to double this skill.
The damage skills arent really interesting, the only serious damage ranger builds rely on a hammer or dual shot + quick/punishing for spikes. For DPS you really need a warrior and leave the uttility skills in the ranger's bar.
Dodge migt be usefull on a runner, for flag runners this isnt to significant as there are a lot of other interesting skills that you'd like to fit on your bar so you'll end up with one normally anyway. For PvE runners this doesnt matter to much to me either as any decent runner can make the run anyway.
Monk Skills:
Imho all the monk skills are either spamable allready or not worth it anyway. So no balance issues here either.
Necro Skills:
Next to the touch skills I dont see too much problems occuring here.
On the touch skills, some problems might occur here, a ranger/necro with two vampiric touches, a touch of agony (second one shouldnt be necesary due to recharge times and a bit of walking between activating) and offering of blood can go for a continuous chain of these skills and provide an insane armorignoring damage output while getting a huge selfheal.
Mesmer Skills:
Here two skills might prove unbalancing for PvP and one for PvE.
In PvE it's now easiely possible to keep up SV permanently, providing huge advantages for a lot of possible farm enviroments where melee attacks are combined with otherwise lethal skills (55 monk queen farming while stopping vampiric touches).
In PvP Inspired hex might prove to give some inbalances, with moks running two of these for energy management, what all of a sudden becomes a viable alternative in 8 vs 8 matches.
With echoed + arcane echoed arcane thievery/larcency it will be possible to disable an average of 3.5 spells on a character. Assuming a regular boon prot and 4 spells disabled this leaves the boon prot with the signet of devotion (which could be disabled as well) + 3 spells. With a bit of bad luck this completely eradicates the monks healing cpacities (e.g. being left with boon, CoP and veil). This depends a bit on lcuk but in extended battles (like GvG) luck is also the chance you can wait for.
Elementalist:
I don't see too much issues arising here, but I'm not used to playing ele so I might miss a possible glitches here.
Finaly I'd like to end with what I started with, please leave this a thread for discussion on possible imbalancing issues rather than "A-net ***** for copying skills". Thanks once more for doing so.