A person who is praying without a minyan does repeat the last line of the Shema, and even if you are is praying with a minyan but at your own pace, you also repeat the last line. However, if you just have not caught up with the congregation, stop and wait while the Chazzan says the last line, and then you do not have to repeat it. In the Kriat Shema before sleep and when donning Rabbeinu Tam tefillin, you add the word ’emet’ to the Shema but do not repeat the last line.