KB33284: “This key is already associated with an element of this collection” error message appears when trying to save a group form in the Attribute Editor in MicroStrategy Developer 9.4.x-10.x.

SUMMARY:
This knowledge base article explains why “This key is already associated with an element of this collection” error message appears when trying to save a group form in the Attribute Editor in Strategy Developer 9.4.x-10.x.
SYMPTOM:
When creating or editing an attribute with a grouped form mapped to a logical table, the following error message is displayed:

'This key is already associated with an element of this collection'.

• Create a logical table defined as ‘LU_DIST_CTR_LV’ using the columns from the 'LU_DIST_CTR' table using the Add button as shown in the image below:
  

• Create a new attribute with two expressions in the ID form. One expression should be pointing to the physical table while the other one points to the newly created logical table as shown in the following image:
  

• Create a second attribute form and attempt to group the forms. The following warning is displayed:
  

• 
  The category selected for this form is already being used by another form in the current attribute. The only way to have multiple forms using the same form category is by creating a form group. Do you wish to create a form group?

  
  
• ​Clicking on the "Yes" button causes the following error message to appear:
  

• 
  Unexpected error
  This key is already associated with an element of this collection

  
  

CAUSE: 
The column object created in the logical table was defined by typing the expression instead of mapping it to an existing column.
 
ACTION:
Define the column objects in the logical table by dragging existing columns from the physical source table from instead of using the 'Add' button as shown in the image below.
 

 
By following this procedure, users will notice that the attribute can be defined using only one expression because the columns are not duplicated. Attribute forms can be then grouped correctly as shown in the image below:
 

For more information about Logical Tables (Logical View), please refer to the following Technical Notes: 
TN: 12000 (KB5300-800-0666): After creating a column in a logical table, automatic mapping does not occur for attributes referencing columns with the same name in Strategy 9.x  
TN: 16858 (KB5700-8X-2746): How to create logical tables without creating duplicate columns objects in Strategy Desktop 9.