I'm new to 8th edition 40k and as far as I understand you no longer receive the extra attack for 2 melee weapons. But the Death Guard codex gives the option: "Any Plague Marine can replace their boltgun with either a bubotic axe or a second plague knife."
Is there any bonus to taking a second plague knife or a second identical melee weapon in 8th Edition?
You'll want to look for the Vectors of Death and Disease special rule which suggests that amongst it's other effects, having two plague knives increases the bearer's attacks characteristic.
@aslum is correct, having a second plague knife (or a bubotic axe) increases the models attacks by one. As I understand it this extra attack can be made with any weapon the model has (i.e. if you wanted to you could use the extra attack from the axe as a knife attack).
I think that taking extra melee weapons has no benefit RE number of attacks unless it says so in the data sheet or weapon rules. It does allow you to a choice of weapon to use. For example a model with a power fist and a close combat weapon might not want the -1 to hit when fighting a horde of poorly armoured enemies.
