You don't need to == true.

This, I think, can be reduced to:

    ...
    keys := make(map[string]bool)
    result := []*ec2.Tag{}
    for _, entry := range tags {
        if _, value := keys[*entry.Key]; !value {
            keys[*entry.Key] = true
            result = append(result, entry)
        }
    }    
    return result
}

but untested - I just typed it out!

Please could you also create a unit test for this function as it it generic.

This, I think, can be reduced to:

    ...
    keys := make(map[string]bool)
    result := []*ec2.Tag{}
    for _, entry := range tags {
        if _, value := keys[*entry.Key]; !value {
            keys[*entry.Key] = true
            result = append(result, entry)
        }
    }    
    return result
}

but untested - I just typed it out!

Please could you also create a unit test for this function as it it generic.

The length doesn't guarantee correctness. Better to have expected and actual and use reflect.DeepEquals to verify the result.

Add the empty case (i.e., empty slices).

I think it would be more succinct to reduce all of this output to just:

t.Errorf("test #%d: expected %+v, got %+v", c.expected, actual)

as the inputs are quite small.

Can't we just put all the tags under a map and then check if the key already exists?

tagMap := make(map[string]struct{})
for _, tag := range machineProviderConfig.Tags {
 		tagMap[tag.Name] = struct{}{}
}
if _, exists := tagMap["clusterid"]; !exists {
 		tagList = append(tagList, {Key: aws.String("clusterid"), Value: aws.String(clusterID)}}
}
...

personally I like current implementation. It prevents also from injecting duplications @frobware wdyt?

I kind of prefer the de-dupe phase. It's easier to rationalise. I have a bunch of tags, then a last action is to de-dupe them.