I am a Canadian. I will be studying in the UK this fall. The question on the visa application specifically asks "Have you visited any other countries in the past 10 years?" It then ask for the date and length of travel. If I have visited a country more than once, do I say so, or do I just record it once?
You should state the date/duration of every visit in the past 10 years (or up to what is specified in the latest guidance and/or allowed in the system), even if they were to the same country.
The guidance notes to complete form VAF2 (the paper equivalent of your online Tier 4 visa application form, now only available to North Korea residents) says the following:
7.2 Have you travelled outside your country of residence, excluding the UK, in the last 10 years?
Answer Yes/No. You should include travel for study, training, business trips etc. When giving this information please provide details of all your trips abroad that you have not described in answer to question 7.1.
Hence they are expecting the list of trips, but not merely the list of countries you have visited.