2018 tax law for reimbursed expenses related to moving. Box 12a "P" vs letter "L"????

My employer provided W2 with a box 12a "P" for moving expenses. I heard that moving expenses are taxable in 2018 but only if they are part of an "unaccountable plan." Does this apply to reimbursements that I paid for and have receipts for and that were reimbursed? If so, am I getting double taxed on box 12 P for "reimbursed" moving expenses now? 

I paid X amount from my already taxed income to pay for a moving company. My employer repaid me the expense and put it in box 12a as a P. Under the new tax law is this now taxable income? So am I getting taxed on already taxed income that I used to pay for moving and got reimbursed for by my employer? Or does the employer have to change box 12 a from P to L? 

For an example's sake. I paid $5000 of my taxed income to pay the movers. I provide the receipt to my employer. The employer repays me the $5000. Is that now taxable income? Am I getting taxed on reimbursed taxed income that was used for moving???? So confused please help!