Remove Duplicates From Arrays in Power Automate — 3 Approaches

Power Automate still does not have a built-in "Remove Duplicates" action. Here are three reliable approaches, from simplest to most flexible.

Approach 1: Union with Itself

The simplest one-liner. The union() function returns distinct values:

@union(variables('myArray'), variables('myArray'))

This works perfectly for arrays of simple values (strings, numbers). It compares values directly and removes duplicates.

Example:

Input: ["apple", "banana", "apple", "cherry", "banana"]

Expression: @union(variables('myArray'), variables('myArray'))

Output: ["apple", "banana", "cherry"]

Use a Compose action with this expression, and you have your deduplicated array in one step.

Approach 2: Select + Union for Object Arrays

When dealing with arrays of objects and you want to deduplicate based on a specific property:

1. Select — extract the key field into a simple array:

   @item()?['email']
   

2. Compose — deduplicate with union:

   @union(body('Select'), body('Select'))
   

3. Filter array — filter the original array to keep only the first occurrence of each key:

   @equals(
     indexOf(outputs('Compose_Unique_Keys'), item()?['email']),
     indexOf(outputs('Compose_Unique_Keys'), item()?['email'])
   )
   

Wait — that always returns true. The trick is slightly different. Use a Select on the unique keys and for each key, use first(filter(...)) to grab the first matching object:

@first(filter(variables('myArray'), item()?['email'], items('Apply_to_each')))

Approach 3: XPath for Complex Deduplication

For more complex scenarios, convert to XML and use XPath's distinct-values:

1. Compose — convert array to XML:

   @xml(json(concat('{"root":{"item":', string(variables('myArray')), '}}')))`
   

2. Compose — apply XPath:

   @xpath(xml(outputs('Compose_XML')), '//item[not(. = preceding-sibling::item)]')
   

This is more verbose but handles edge cases that union() doesn't cover.

Which Should You Use?

Scenario	Approach
Simple string/number arrays	union() — one expression
Object arrays, deduplicate by key	Select + Union + Filter
Complex nested deduplication	XPath
Performance critical (1000+ items)	union() or move to a child flow with chunking

Performance Note

For arrays with more than a few hundred items, all three approaches work fine. Above 5,000 items, you may hit expression evaluation limits. In that case, consider:

• Processing in batches using a child flow
• Moving deduplication logic to a SQL query or Dataverse view before pulling into the flow
• Using a custom connector with an Azure Function for heavy data processing

Key Takeaway

Start with union(array, array) — it solves 80% of deduplication needs in a single expression. Only reach for more complex approaches when you are working with object arrays or need key-based deduplication.