You can use "in" operator for that in your expression, so the formula will go like:
Filter(DataSourceName, "Value1" in ColumnName.FieldName)
Here, ColumnName is the name of column in SP list which hold multiple values and FieldName is the name of column for that lookup. Value1 is the value that you want to search. If you need more help with the specifics, please share the details about the schema of the list and we might be able to help you better.
Hope this Helps!
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