For the second year in a row, D.C. City Council member Marion Barry (D-at large) has introduced legislation that would guarantee 12,000 city police, firefighters and teachers the same percentages pay increase in October as other municipal and federal employees will get.

Barry's action, which he announced at a new conference yesterday, was done at the request of one of the city's two rival police unions, and won strong support from three city employe unions.

Union leaders said introduction of the bill does not automatically assure Barry of their endorsement in his campaign for the Department nomination for mayor. But, added David A. Ryan, president of the Fire Fighters Association, "It sure does not work against him."

The measure was cosponsored by Council member John A. Wilson (D-Ward 2).

If the measure passes, it would be the third year in which the council has tied police and firefighters wage increases to those granted so-called general schedule (GS-rated) employes. Last year, teachers were added to the police and firefighters' bill, with Barry as the principal sponsor.

A law passed by Congress provides for setting police and firefighters' pay by negotiation between the unions and the executive branch of the city governments - a process union leaders had hoped would produce larger wage increases for their members than the other employes get.

However, the negotations bogged down the first time they were attempted, and the council stepped in to grant the same cost-of-living increases as all D.C. and U.S. government GS rated employes received.

The amount of the increases is recommended each fall by a panel chosen by the White House. Last year's increase was 7.05 percent. Barry predicted this years's increase would be about 6 percent.

The future pattern of the D.C. government's pay system is uncertain. The council is working on legislation to create a new municipal personnel system detached from the federal civil service. Council leaders hope to have the measures enacted later this year, eliminating the need for bills such as Barry's.

Unions represented at yesterday's news conference, in addition to the Fire Fighters, were the International Brotherhood of Police Officers (which proposed the Barry bill) the rival D.C. policemen's Association and the Washington Teachers Union.

A spokesman for Mayor Walter E. Washington said the mayor had no comment on the Barry proposal. The mayor, who is expected to seek the Democratic nomination to suceed himself, was home for the fifth day with what was described as a severe case of flue.