Same issue here on self-hosted email account (version MIUI_V11_EMAIL_20191206_b1). Has been happening since ROM version 9.12.19 trough to 9.12.26 on XIaomi Mi 9 SE. Looks like the INBOX folder gets duplicated every second or so, so the app keeps spamming notifications for the same email but in the newly created INBOX.
EDIT: downgrading to MIUI_V11_EMAIL_20191024_b4 fixed the issue for me so this is definitely a bug. :/
EDIT2: MIUI_V11_EMAIL_20191206_b2 is still broken with the same issue.