Concentration is the deliberate or accidental clustering of risk. It is how fortunes are made and lost, which is why the debate about it never ends.
Quantify rather than argue. If one position contributes 40% of portfolio-volatility, the portfolio is that position plus noise. A simple test: compute what a 20% adverse move in your largest holding does to total equity. If the answer exceeds a typical month's profit, the holding is oversized regardless of conviction.
There is a legitimate case for concentration when you have genuine informational edge and a long horizon, and no case at all when it arises by drift. The dangerous form is hidden concentration: ten names, one factor. See effective-number-of-bets for the measure that exposes it.
Related: sector-exposure, single-name-limit, diversification