Neil Walker was traded from the Mets for a chance to play meaningful games over the final two months of last season, but now team officials are wondering if he might consider a reunion. The free-agent infielder remains high on the Mets’ list of possibilities to fill their opening at second base, according to industry sources, but in this case, familiarity may hurt the club more than benefit it. At issue is the failed contract-extension negotiations between Walker and the team from earlier this year, causing bruised feelings that could leave the player and his representatives hesitant to embrace the possibility of a return. The Mets are exploring various options at second base — trades among them, with names such as Jason Kipnis, Ian Kinsler and Dee Gordon available — but signing a free agent would allow the team to preserve trade chips that might be needed to address other areas. Walker, 32, accepted a qualifying offer from the Mets worth $17.2 million before last season, and the two sides worked toward an extension before negotiations broke down early in spring training and never regained serious momentum.