Allow directive is a non-standard extension to robots.txt. The original robots.txt standard only includes
Disallow directives. Google's robots do understand
Allow directives, so it would be OK to use them in sections targeted at a Google robot. The
Allow directive should always be used after a
Disallow directive to counteract the
Disallow. It can be used to give finer grained allowance (maybe just a single page) of entire section that would otherwise be disallowed.
In your case, the
Allow directive is saying the same thing as the
Disallow directive, so it may be omitted.
You say you that you want to allow all Google robots to crawl all of your site. If that is the case, you would need a rule for the main
Googlebot as well. I would suggest using this in your
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