The number theory work of Fermat might be an example. He was rather secretive about his methods and much has to be rediscovered later by Euler. This includes Fermat's two-square theorem: It was first mentioned by Fermat as a theorem in a 1640 letter to Mersenne and also analogous statements about primes numbers of the form $x^2+2y^2$ and $x^2+3y^2$ were made in a 1654 letter to Pascal. While Fermat claimed to have solid proofs, he did not write more than a very vague sketch using infinite descent. Euler first became aware of Fermat's work around 1730. It took Euler until 1749 to prove Fermat's two-square theorem and until 1772 to prove the analogous statements about primes numbers of the form $x^2+2y^2$ and $x^2+3y^2$.

A more knowledgeable person could certainly present more examples in the work of Fermat and Euler. I do not include "Fermat's last theorem" as it seems virtually impossible to me that Fermat possessed a correct proof for this.