With an Office 365 subscription, Excel provides additional formula functions, perfect for this exact task.

=TEXTJOIN(" ",TRUE,UNIQUE(TRANSPOSE(TEXTSPLIT(A1," "))))

This is identical to @user11308575 answer, except using Excel syntax rather than Google Docs syntax.

Personally, I used this with non-space delimiters (","), but to answer the question correctly, the formula was written this way.