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"Mermaid Knight" can attack twice if "Maiden of the Aqua" is making the field treated as "Umi".
If you declare your second attack with "Mermaid Knight", and "Umi" is destroyed during the Battle Step, a replay occurs and you cannot attack a second time with "Mermaid Knight".
If there is no "Umi" on the field and "Mermaid Knight" attacks, then you Special Summon "Maiden of the Aqua" to treat the field as "Umi", you can attack a second time.