resolve duplicate topic tags

[yangmarc]

Issue #, if available:

Description of changes:

This PR resolves the duplicate tags if same tags with different value. Previously, it will cause failure.

The resolve rule is that last tag wins.

Tested with:

{
    "Resources": {
        "MyTopic": {
            "Type": "AWS::SNS::Topic",
            "Properties": {
                "Tags": [
                    {
                        "Key" : "keyname",
                        "Value" : "value1"
                    },
                    {
                        "Key" : "keyname",
                        "Value" : "value2"
                    }
                ]
            }
        }
    }
}

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