Reject duplicate entries in SUMMARY.md

[szabgab]

Problem

If the same md file is mentioned more than once in the SUMMARY.md they will still generate the same html file so the prev and next links will lead to the same page. From the point of view of the hierarchy this will seen to be "broken".
See #1629

- [A](./a.md)
- [B](./b.md)
- [A](./a.md)
- [C](./c.md)

Should the next link in a.html lead to B or C? Both will be considered broken.

Proposed Solution

IMHO mdbook should report an error if the same file is mentioned twice.

Notes

No response