Throw better error on duplicate frontmatter key

[martinjagodic]

Is your feature request related to a problem? Please describe.
Sometimes we commit frontmatter via git and it happens that we duplicate a key. This causes an error, but it's not clear at all what's the cause.

Describe the solution you'd like
Show a better error: which key in which file is duplicated. Possibly propose a fix to delete the extra line. If the lines are different, offer which one to keep.

Describe alternatives you've considered
If you somehow realize that keys are duplicated, you need to find and remove them, which can take some time.

Example

---
title: Example post
description: Hello world!
title: This second title causes an error 
---