I am worried I may be penalised by Google for content duplication
There's not really a "penalty" as such, however, what you will find is that if Google does perceive these URLs as duplicate then it will only return one of the them (for any given page) in the search results. It may even favour one domain over all others - if it sees that domain as authoritative.
However, it's also confusing for users that might stumble across the different sites and realise the content is the same.
What way would you go about doing this?
If you are intending these domains to be indexed then that's a tricky one. Ordinarily, in order to avoid any duplicate content issues, you'd pick one as the "canonical" domain and redirect to that. (Or, set a cross-domain
rel="canonical" meta tag, or HTTP response header? At least that way, the other domains would be accessible.)
If you don't take manual action in resolving what is potentially duplicate content then Google will simply decide for you, if it is perceived as duplicate. Possibly the page with the most quality backlinks will "win"? The same page on the other domains may not appear at all in the SERPs - that is the duplicate content "penalty". Google is trying to return quality results to its users. Returning 3 or 4 pages that contain essentially the same content (Google doesn't really care about "branding") is not "quality search results" - so it is something that Google actively tries to avoid.
Your domains are going to be competing against each other. The only way to avoid the "duplicate content" issue is to vary the content.