I get the notification twice for the achievement and now I've 2 Miniature Vending Machines in my inventory. what should I do? simply delete one? send a ticket?

Leave the two Vending Machines in your inventory and make a ticket so that a User Admin can look into it and delete one of them. It's always better to let staff know about duplicated items and the like.

thank you

I would send a ticket about this for sure. I had something similar a bit ago where I received two lifelike dolls at the same time, I filed a ticket and the assigned staffer did their thing. I'm sure this is something you could just delete yourself but it could also be something staff needs to look into, error wise.

[edit] wow, did that really take me at least 6 minute to write? a good sign i should probably sleep. ;P

This seems to be an issue. You're not the first one to ask about it. I would file a ticket for the admin to delete the duplicate.