Every now and then you will come across a once in a lifetime investment - that not only feels too good to be true, but also has all the hallmarks of being a true winner. On these occasions, it is very hard not to end up in a situation where one share dominates the rest of your portfolio - and if that share goes on to do well, then of course this is a great result - but the focus on a single share will increase it's dominance on overall performance. These days my largest holdings (based upon current market values) are as follows: 19.2% in a market index (FTSE 100) 18.9% in a single share (hopefully a once in a lifetime winner) 6.6% in a market index (FTSE 250) 3.6% in a single share 3.3% in a single share When it comes to investing there are two schools of thought: Diversify across a mix of shares, ideally via index trackers Focus on high-convention bets where you believe there is low risk and high reward In reality, many of the best investors use a mix of enough diversification - b...