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It could be a sign that the brake light bulb is on its way out. For the sake of it I'd be tempted to replace it.
Or to investigate you could swap them over between sides and see if the computer tells you the other side is faulty. That way you will know if you have a bulb or a cluster earth problem.
Currently in dave_mezza2006's garage:
2014 Vauxhall Astra Limited Edition 1.4 16v VVT Turbo