Internal linking
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Do you have to be careful spamming internal links?
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Short answer yes.
Google will only read 'x' number of links on a page anyway so too many and they wont even be read by Google. Too many internal links, like a link farm and this will be penalised.
Also I would avoid keyword stuffing in your anchor text, as this will look spammy.
Great article here on internal link building http://www.quicksprout.com/2014/05/14/how-to-avoid-getting-slaughtered-by-penguin-3-0/
Internal links is great way of helping your users navigate around the site, but use internal links to help people navigate your site and not purely for SEO. If you do it naturally and forget about the search engines you will be fine, its when you try and cheat them it will look spammy and you have the potential to be penalised.
When adding a link, ask yourself: Does the user need this link, will it help them on their journey.