How does having duplicate content on your domain give you a spamming penty? And for a site thats been a round for a while only having 100 404's is low - most s

How does having duplicate content on your domain give you a spamming penty? And for a site thats been a round for a while only having 100 404's is low - most site owners are v bad at tidying up old defunct urls and using script kiddie platforms like joomla and magento dont help.