The label from the Library.xml file supersedes the label in the individual application design.
For example, if there is a textbox which has the ID="new_tablecol7" in the Library.xml and it has a label value of "Priority", then the designed label in that application is replaced with the value "Priority" in any application where there is a textbox with the ID="new_tablecol7".
This supersedes the individual application design.
Resolving the problem
This problem can be fixed if the ID is renamed to a different value in the library.xml file, but it appears that Application Designer should either allocate unique IDs or that it should know which one should take precedence: the library.xml or the application XML.
Please note that Application Designer was enhanced to use unique IDs when adding new controls from Maximo 184.108.40.206 onwards.
The only way to avoid the issue in Maximo 6.2.x is changing the ID of the control in question.