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Hello,

(a). A process that leads to the occurrence of one and only one of several possible observations

(b). A process that leads to the occurrence of only one of several possible observations

The sentence (a) is the original sentence.

(1) What is the difference in meaning between sentence (a) and sentence (b)?

(2) Is the sentence (b) grammatically correct?

(3) Does sentence (b) convey the same meaning as sentence (a)?

(3) In what context, 'one and only one' must be used?
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1 No significant difference. Emphasis on singularity.
2 Yes. b is correct.
3 Yes.
4 There is no context in which it must be used. It can be used whenever you wish to draw attention to a singularity. It is used most often when the author believes that the typical reader would not normally pay enough attention to this singularity to understand the point which is being developed in the text.

CJ
Comments  
Thanks much for a quick and clear explanation, CJ.