BUG: interpolation with duplicate index values fails (#42585)

[bkurtz]

• closes BUG: interpolate method with 'index' and 'slinear' methods give inconsistent results when there are duplicates in x-axis #42585
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

[bkurtz]

working on adding test cases here, and suspicious I may have missed a couple of details. No rush to review until I get tests added.

[github-actions]

This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this.

[mroeschke]

Thanks for the PR, but it appears it has gone stale. Closing for now, but if you could merge master and try to get the tests passing locally we can reopen.