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The NA62 experiment reports an investigation of the mode from a sample of decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of , corresponding to 2.2 events assuming the Standard Model branching ratio of . Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of for the branching ratio at 90\%\,CL. This, together with the corresponding 68\%\,CL measurement of , are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements.