Final answer:
The main difference between $push and $addToSet group accumulators is that $push allows duplicates in the resulting array while $addToSet ensures each value in the array is unique by preventing duplicates.
Step-by-step explanation:
The difference between the $push and $addToSet operators used in MongoDB group accumulators is in how they handle duplicate values when grouping documents. The $push operator adds the value to an array regardless of whether the value already exists in the array, effectively allowing duplicates. In contrast, the $addToSet operator adds the value to an array only if it does not already exist in the array, preventing any duplicates.
For example, if we're grouping by a field and accumulating values from another field, using $push would result in an array with all values collected from the grouped documents, including any repetitions. The $addToSet would result in an array where each value appears only once, as it filters out any repeated occurrences.