#221 Check for duplicates functionality

[m-yoda]

It is very useful to have a way to check for duplicates inside of a lesson file. I see two possibilities at least:
- propose duplicates during the Card save operation (like search)
- search for duplicates as a separate function callable from the main menu

Reason:
- I currently learn a foreign language and I've noticed that some words I created two times and this happens since the words I do not know yet :-)

[m-yoda]

For speeding up the duplicates identification, I would suggest searching across all categories (by default) and only inside of a category (configurable).