wxGUI/dbmgr: don't add new duplicated table col into table desc TableListCtrl widget

[tmszi]

Describe the bug Duplicated column name be added into table description TableListCtrl widget (see screenshot below).

To Reproduce Steps to reproduce the behavior:

1. Launch Attribute Table Manager e.g g.gui.dbmgr geology
2. Switch to Manage tables page (tab)
3. Add new column with same name as existed col name GEO_NAME
4. You see correct error dialog with message: Column <GEO_NAME> already exists in table .
5. Duplicated column name is added into table description TableListCtrl widget

Expected behavior Duplicated column name shouldn't be added into table description TableListCtrl widget

System description (please complete the following information):

• Operating System: all
• GRASS GIS version: all