I'm not totally sure why one would want to center the last line in a paragraph, but the excellent TeX by Topic provides a surprising solution.

\leftskip=0pt plus.5fil
\rightskip=0pt plus-.5fil
\parfillskip=0pt plus1fil

The reason this works is that for all lines except for the last, the amount of stretch in the line is 0, so no skips are inserted. In the last line, the addition of the \parfillskip gives a skip of .5fil on the left and right so the line is centered.

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