Execute failed: Duplicate column name

hi im using knime and i always get this error  Execute failed: Duplicate column name  when using nueral network how can i solved please 

What about using the Column Rename node?

i used it it diddnt work 

this is my work flow 

this is the image of it 

You have 4 classes in your data "low" "medium" "high" and "high ". Some of the "high" classes have a space after the word. Please clean up your data and try again.

You can use the function strip() in the String Manipulation node to clean up extra spaces from strings.

i try it  it get me me the erorr WARN  String Manipulation  3:12       failed to apply settings: Unable to compile expression
ERROR at line 59

what did you enter in the dialog?

Maybe our online training would be a good start for you: https://www.knime.org/knime-introductory-course