PlanGuru does not restrict you from adding duplicate account numbers or descriptions. I think we can all agree that having unique account numbers (and descriptions) is best practice. That said, when we designed PlanGuru we didn't feel that such a restriction was required.
Its important to note that having duplicate account numbers or names will cause issues when retrieving results through the Advanced Reporting Add-In. I'll illustrate with the following example:
In the below example the user has two accounts with duplicate account numbers. When the customer sets up a report using the advanced reporting add-In, and tries to pull results for account "5000", they will receive the sum of both accounts identified by the "5000" account number.
In this same analysis the user also has two accounts that have duplicate account descriptions. Thus if the user is retrieving results by account description and attempts to run a report for the "Cost of Sales type 1" account they will get the sum of the accounts with this description.
The resolution for each of these issues is to make the account numbers, or descriptions (preferably both) unique. Here is the rule of thumb, because the Advanced Reporting Add-in allows you to pull results both by account number or description you should decide which one you will use in your reports. If you need to pull values by account description, then your descriptions should be unique within your PlanGuru analyses. If you need to pull values by account number then your account numbers should be unique.