MongoImport: problem with Duplicate Key

[bitliner]

What happened is the following:

• I exported some documents using mongoexport
• I removed the same documents that I exported
• I tried to import the exported documents using mongoimport

The import fails because it throws "Duplicate Key" error.

But the documents that I am trying to import are not in the collection, so there is no duplicate _id.

Could it be that there is some cache or index that keeps them and make the database throw the error?

There aren't other unique indexes apart of _id.

If I import the documents in another collection it works fine.