As far as I know, we should omit the pronoun in the relative clause when the relative pronoun refers to it.
For example, we should say:
1 One of the most paramount features of today's life is the Internet that people cannot dispense with.
And not say:
2 One of the most paramount features of today's life is the Internet that people cannot dispense with it.
However, I want to use the following sentence in a formal writing and I couldn't decide whether I should obey this rule in it or not.
3 These days, so widespread is the Internet that people cannot dispense with.
4 These days, so widespread is the Internet that people cannot dispense with it.
Would you please explain the wisdom behind using or not using "it" here so that we, English learners, can generalize the answer of this question to other similar situations?