When the first leap seconds were added, which I believe were in 1973, there were two inserted at the same time. Thus a single legal value has had the seconds denoted as "61". From memory, these two leap seconds were:
1973 June 30 23:59:60 1973 June 30 23:59:61
No, only one second is ever added at a time. However, Ron is right about the first year (1972) containing two leap seconds. Below is the relevant section describing how leap seconds are inserted/deleted. Bob Devine [ On a sidenote to time, the Herstmonceux Castle where the Greenwich Observatory is located is for sale. You can buy it for only ~$15M.] From the NBS Special Publication #432 (issued in 1979): When leap second adjustments are necessary to keep the broadcast time signals (UTC) within +/- 0.9 second of the earth-related UT1 time scale, the addition or deletion of exactly 1 second occurs at the end of the UTC month. By international agreement, first preference is given to December 31 or June 30, second preference to March 31 or September 30, and third preference to any other month. When a positive leap second is required -- that is, when UT1 is slow relative to UTC -- an additional second is inserted beginning 23h 59m 60s of the last day of the month and ending at 0h 0m 0s of the first day of the following month. In this case the last minute of the month in which there is a leap second contains 61 seconds. Assuming that unexpected large changes do not occur in the earth's rotation rate in the future, it is likely that positive leap seconds will continue to be needed about once per year. If, however, the earth should speed up significantly at some future time, so that UT1 runs at a _faster_ rate than UTC, then provision is also made for negative leap seconds in the UTC time scale. In this case, exactly one second would be deleted at the end of some UTC month, and the last minute would contain only 59 seconds. Positive leap seconds were inserted in all NBS broadcasts at the end of June 30, 1972 and December 31, 1972 thru 1978. [ All years since 1978 have had leap seconds. Bob]