If a donation in Legacy RaiseDonors shows an error from Authorize.net similar to, Error 11: A duplicate transaction has been submitted that means a transaction with an identical amount and credit card information was submitted within the Duplicate Transaction Window (exact details on what Authorize.net looks to compare here). The error indicates that a transaction request was submitted within a few minutes of a previous attempt with the same information. Authorize.net identifies duplicate transactions by matching the data provided with the transaction.
You need to modify the "Duplicate Window" setting in Legacy RaiseDonors. To do so, log into Legacy RaiseDonors and go to your merchant settings page. From there, supply a lower or higher value in the "Duplicate Window" section.
The values in the "Duplicate Window" section are measured in seconds, and the largest allowed value is 28800 (8 hours). While you can manually enter an amount greater that 28800, the payment gateway will default to 28800. If the amount is set to 0 or to a negative number, there will not be a duplicate transaction window enforced in Authorize.net. If the field is left blank and no value is set, the default value of 120 (2 minutes) will be used.
For example, if you wished to set a duplicate transaction window of 5 minutes, you would set duplicate window to 300 (as there are 300 seconds in 5 minutes).
Note - if you are in a situation to manually create recurring schedules, having multiple schedules per donor, there is logic within Authorize.net that checks for duplications. These settings are unfortunately, not customizable.