Onur took the work seriously and Petro took it even more seriously.
As far as "not" goes, do you mean that you want to say:
Onur took the work seriously and Petra perhaps even more not?
If that is what you mean, then no. The point of the sentence is that Petra not only took the work AS seriously AS Onur, but in fact MORE seriously. If Petra didn't take it seriously, then:
Onur took the work seriously but Petra didn't.
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