Let me describe 4 possible cases that may happen when fighting on Coalition Arena.
Case 1
A player A clicks on the 'Attack Warlord' button. The game shows a window where he/she can select a Guardian. Meanwhile, a player B sends a Guardian to the same Warlord and kills it.
Result
The moment when the player A selects a Guardian for an attack, he receives a notification that the Warlord is already defeated, and the player A can't send his Guardian. The energy from the player's A account isn't deducted, the specialization of his Guardian isn't frozen.
Case 2
A player A clicks on the 'Attack Warlord' button. The game shows a window where he/she can select a Guardian. Meanwhile, a player B sends an attack to the same Warlord.
Result
The moment when the player A selects a Guardian for an attack, he receives a notification that another player has already initiated an attack against this Warlord. The energy from the player's A account isn't deducted, the specialization of his Guardian isn't frozen.
Case 3
A player A clicks on the 'Attack Warlord' button. The game shows a window where he/she can select a Guardian. Meanwhile, a player B sends an attack against the same Warlord, does damage, but doesn't kill it.
Result
The player A sends a Guardian and kills that Warlord. The energy is deducted from the player's A account, the specialization of his Guardian becomes frozen.
Case 4
A player A clicks on the 'Attack Warlord' button. The game shows a window where he/she can select a Guardian. Meanwhile, a player B sends an attack against the same Warlord, too. The player A manages to send his Guardian. Despite the fact that the Guardian is already on the way for some period of time, in fact, the first hit is done by the player B.
Result
The player A receives a message from Leonidas informing that the battle attempt hasn't taken place, because this Warlord is already under an attack. The energy from the player's A account is deducted, however, the specialization of his Guardian won't be frozen in this case.
ARES, it seems to me that you describe the 3rd case, however, if I'm mistaken, I'll need to have screenshots to check it. If you or any other player face this issue again, please take screenshots and send them in reply to this thread. Thank you.