<issue_start><issue_comment>Title: Duplicate value for merged cell instead of `None`
username_0: See #420
<issue_comment>username_0: I saw #387:

- [ ] That PRs should be submitted against develop, not stable
- [ ] That PRs should contain tests
- [ ] How to write new tests
- [ ] How to run tests
- [ ] How to run linter and reformat code

- [ ] Adding oneself to the list of contributors
- [ ] Update CHANGELOG.md
<issue_comment>username_1: Hi @username_0, thanks for your interest in this library and thank you for the PR. I agree that `pdfplumber`'s current approach to complex tables is not optimal, but it is the result of not wanting to over-impose assumptions about any given table. The handling of merged cells in tables is a tricky topic, and one with a lot of edge cases — what may be a good solution for one set of tables may be a poor one for others. So I'll need to consider this suggestion and PR somewhat carefully, and think about (and test) other PDFs.

Relatedly: I have an idea for providing a richer representation of complex tables — one that would make the structure of the tables much clearer — but it would be a substantial change and so it will probably have to wait for `v0.6.0`.
<issue_comment>username_0: Thank you for the consideration, @username_1. Please let me know if you found bug with my PR. I'm happy to provide fix.