To what extent similar remarks from the same author can appear in different papers?
To some extent plagiarism isn't charged when there is essentially only one way to say something. But this rule mostly applies to things that are widely known, not research results. This may be the case here or not. 

But it is better to cite the other (first) work in subsequent work (or cross cite if appropriate). One of the purposes of avoiding plagiarism is to allow readers to have access to the complete context of an idea. This might be especially useful in the case at hand, since the "similarity" itself may provide context that would be helpful. 

When in doubt, cite it.