can I have the same company name as an existing company?

It depends on whether the other company's name has achieved trademark status, which it can do even if it is not registered as a trademark (and conversely you can challenge even if it has been registered as a trademark).  This is a complicated question, but the most important factor is whether customers are likely to be confused into thinking that your company is either the same as or affiliated with the original company.  For example, you may be able to operate McDonald's auto repair shops (although you probably can't appropriate the golden arches or other McDonald's trademarks or trade dress even in that area) because it is not likely to confuse consumers into believing that your business is affilaited with the fast food restaurant chain, but you probably can't operate McDonald's luncheonettes, and you certainly can't operate McDonald's hamburger restaurants.