Thank you for your post.
I am unable to replicate the issue.
In FileMaker Pro 12.0v5, I created a database with several number fields, created a line chart with six line graphs. I then opened the file in FileMaker Go 13, and the six line graphs are displayed. I did not get any duplication.
If you create a new file with similar values, are you able to replicate the issue? If so, please send me your database file so I can get try to determine what is causing the issue. Check your Inbox at the top of this page for instructions where to send the file.