In Excel: select the range, then Home ▸ Conditional Formatting ▸ Highlight Cells Rules ▸ Duplicate Values, and pick a fill colour.
Select the cells to check (drag over B2:B20, or click a column header for the whole column).
On the Home tab, open Conditional Formatting ▸ Highlight Cells Rules ▸ Duplicate Values.
Choose a fill (e.g. Light Red Fill with Dark Red Text) and click OK.
To extend the check later, reselect a wider range and reapply, or edit the rule in Manage Rules.
What this does
Conditional Formatting colours repeated cells automatically, giving you an instant visual map of where duplicates sit — no formula and no helper column. The built-in Duplicate Values rule compares every cell in the selection and shades the ones that appear more than once. Because the rule is live, the colours update the moment you edit, paste or add data, so duplicates can never sneak back in unseen. Keep the inputs visible and clearly labelled and the whole thing stays auditable — anyone who opens the file later, including you, can see at a glance exactly what feeds the result and change one assumption without hunting through the formula. For “highlight duplicate cells in excel”, the reliable version is a short checking loop, not just the first command that appears to work. Run it on a deliberately small range first, watch how the affected duplicates change, and only then apply the same setup to the full sheet. When this is a ribbon command, the selection matters more than the button: confirm the range, apply the command, then spot-check the output before saving. That is what makes a data step that keeps your analysis trustworthy useful in real work: repeatable, auditable, and not dependent on memory or luck.
A worked example
You have customer emails in B2:B20. Select that range, go to Home ▸ Conditional Formatting ▸ Highlight Cells Rules ▸ Duplicate Values, and choose the red fill. Every email that appears twice — say jo@acme.com sitting in B4 and B17 — turns red in both cells, while one-off addresses stay white. Type a new repeat and it reddens instantly. When you want to see duplicates at a glance during data entry or review — spotting a double-booked email or a repeated invoice number on screen — coloured cells beat any formula, because the eye catches them instantly and the rule keeps watching as you type. If there is any chance you will reuse this, drop it into a small template tab right now: a labelled input area on the left and the formula beside it, checked once against the tool above. Next time the same question comes up, the answer is a single paste away instead of a rebuild from memory.
In Google Sheets
Google Sheets handles this almost identically to Excel. The formula syntax above is the same, and the menu lives under a slightly different label rather than a ribbon tab. Use the platform toggle at the top of the page to switch every keyboard shortcut between Windows and Mac, and expect at most cosmetic differences in naming. The aim was to get you unstuck fast and leave you a little more capable than a copy-paste would. The answer is at the top, the tool proves it, and the detail above shows why it holds — so the next time a colleague asks, you can answer without reaching for search. The short version of “highlight duplicate cells in excel”: the answer is at the top of this page, the tool proves it on your own numbers, and the sections above explain why it holds so the next variation does not stump you. Excel rewards people who reference cells instead of typing values and who keep inputs separate from formulas, because that is what makes a result you can audit months later. Build it once, deliberately, with the live tool as a check, and you convert a one-off lookup into a reusable skill — which is the whole point of learning the why and not just the what.
Common mistakes
- Selecting one column when duplicates should be judged across several — the rule only compares the cells you highlight.
- Expecting the colour to remove anything; highlighting is purely visual, deletion is a separate step.
- Leaving the selection too small, so duplicates outside the range never get coloured.
- Confusing "Duplicate Values" with "Unique" in the same dialog — the dropdown switches between colouring repeats and colouring one-offs.
Frequently asked questions
How do I highlight whole rows, not just cells?
Use a formula rule instead: New Rule ▸ Use a formula, enter =COUNTIF($B:$B,$B2)>1 and apply the fill to the whole row range.
Can I highlight duplicates in one colour and uniques in another?
Yes — add two rules: one Duplicate Values rule and one Unique Values rule, each with its own fill.
How do I clear the highlighting?
Home ▸ Conditional Formatting ▸ Clear Rules removes the colouring without touching your data.