Two entries called Alex appear in the address book. One is a colleague; the other is a music teacher. A matching name is a reason to compare the cards, not a reason to combine them. Start with a pair you can identify confidently instead of approving every suggestion.
Decide whether the details belong together
Open both cards and compare the numbers, email addresses and labels. Check which account holds each entry. A work address and a personal number might belong to one person, but an unfamiliar number needs confirmation. A matching photograph is not enough either.
On iPhone, duplicate suggestions can be reviewed below My Card in Contacts. Examine the proposed pair individually. If you cannot explain how the details fit one person, leave the cards separate and add a useful distinguishing label where appropriate.
Before a larger cleanup, save an export using the relevant contact service and inspect a few entries in the saved file. Keep that copy somewhere private: an address book contains other people's information too. An export you have never opened is a weak recovery plan.
Check the result where you use it
Linking cards from different accounts can produce a unified iPhone contact without merging the underlying cards. A single visible entry therefore does not prove that there is only one stored record.
After changing one confirmed pair, reopen it in the source account and on another device you use. Check that the personal and work labels still match their numbers. When composing a message later, inspect the selected address before sending; there is no need to contact someone merely to test a cleanup. Only then continue with the next clear pair.