Every deal lives in exactly one pipeline at a time, moving left to right through stages your team defines, like Qualified, Demo Scheduled, Proposal Sent, Closed Won. Each stage has a default probability percentage, used to weight your forecast, and paid tiers let you require specific fields be filled before a deal can advance to a given stage, a basic gate against someone fat-fingering a deal to Closed Won with half the fields empty.
Running more than one pipeline, say a separate one for renewals versus new business, is a Sales Hub feature, not something the free CRM offers; check your current tier if you're trying to split pipelines and the option isn't there.
Merging and importing deals work the same way they do for contacts. A deal's Actions > Merge combines two records, favoring the one you pick as primary. The Import tool takes a CSV of deals and can associate each row with an existing contact or company automatically if the matching data lines up.