I am an employee of a company and pay my appropriate taxes through my paycheck. I also own 50% of an LLC of which I trade securities through. Do I have to pay SE taxes through the LLC even though I already pay the maximum Social Security and Medicare?
Check with your accountant, but I believe the answer is no. When you fill out your Form SE for your LLC, there is a line where you list FICA taxes paid from other sources. When you list the FICA taxes withheld from your day job (lines 8a-8d on Schedule SE to your 1040), those will be subtracted out and you'll owe no additional FICA taxes on your LLC's income.
Remember that Medicare taxes (2.9%) don't have the same wage cap (currently $97,000) as Social Security, so even if you cap out on Social Security taxes at your job, you'll owe Medicare taxes on your income above $97,000. However, you won't have to pay Medicare twice on the first $97,000 of your income, as lines 8a-8d on Schedule SE tax form take the withholdings from your job into account.