Complex sentence: Abolitionists argue that a dividend tax amounts to unfair "double taxation", in the sense that the company has already paid a corporation tax on these profits, which means that the shareholders, as part owners, have likewise been taxed already.
Split into simpler sentences: Abolitionists argue that a dividend tax amounts to unfair "double taxation". Since a company has already paid a corporate tax on these profits, this means that the shareholders, as part owners, have been taxed already.