“Obnoxious Celtic Guard” and “Celtic Guardian” are considered 2 separate monsters so you can have 3 of each in your Deck.
“Obnoxious Celtic Guard”’s effect looks at the ATK during Damage Calculation. So if he’s WIND and is attacked by “Insect Soldiers of the Sky”, their ATK is 2000 during Damage Calculation and “Obnoxious Celtic Guard” is not destroyed. Or if an 1800 monster attacks and you use “Rush Recklessly” to increase the ATK to 2500 during the Damage Step, “Obnoxious Celtic Guardian” is not destroyed.
“Obnoxious Celtic Guard”’s effect looks at current ATK, not original ATK. So a monster with 1000 original ATK equipped with “Megamorph” to make it 2000 ATK will not destroy “Obnoxious Celtic Guard” as a result of battle.
“Obnoxious Celtic Guard” isn’t destroyed by damage calculation but can be destroyed by monster effects like “Ryu Kokki”.
If “Obnoxious Celtic Guard” is attacked by “Dark Ruler Ha Des”, or another monster that negates a card’s effect after it destroys it, “Obnoxious Celtic Guard” is not destroyed (so its effect is not negated).
“Obnoxious Celtic Guard”’s effect works for a battle caused by “Last Turn”, so if it battles a monster with ATK 1900 or higher, neither is destroyed and the Duel is a draw.