"Even in his own lifetime Johannes Brahms was a legendary figure and he behaved accordingly. Nevertheless, he was fumbling for words when in 1874, in the Leipzig Gewandhaus, he had to apologize to his younger colleague and friend Julius Röntgen for 'stealing' the main theme of the latter's Serenade for seven winds, opus 14. The old master from Vienna had come upon the handwritten score of Röntgen's appealing piece for winds some years earlier. Somehow the main theme of the first movement had stuck in his memory and Brahms used it, quite unwittingly, as the main theme of his Second Symphony's opening movement. In later years the anecdote of Brahms's borrowing from his Serenade would be recounted often and with relish by Julius Röntgen."

Has any one else heard about this? The first movement of the Second Symphony is one of the great Brahms's most carefully constructed and organic works and we had thought it totally original.

No, Mrs. Kerfoops, I hadn't heard that but Röntgen is an excellent and underrated composer, IMHO.  His chamber works are especially interesting.  I think I shall put that CD on now, thanks for the prompt!!