I have my Gmail forwarding to my iCloud. Because my replies are from iCloud now, and for some people it's confusing, so they send me emails to both iCloud and Gmail. This means I get two emails in iCloud. Here's a workaround for this issue, if it happens to affect you too. Obviously it can work for Outlook forwarding too.
1. Go to Gmail > Settings > Forwarding and POP/IMAP
2. Disable forwarding.
3. Go to Settings > Filters
4. Add a new filter
5. Put "-youraccount@icloud.com" in the To field, without the quotes but including the minus sign. This means any emails you receive in Gmail that aren't also being sent to your iCloud account.
6. Click Continue and select 'Forward it to': youraccount@icloud.com
7. Tick anything else you might like, such as 'Mark as read'
8. Save
9. Test it out!
1. Go to Gmail > Settings > Forwarding and POP/IMAP
2. Disable forwarding.
3. Go to Settings > Filters
4. Add a new filter
5. Put "-youraccount@icloud.com" in the To field, without the quotes but including the minus sign. This means any emails you receive in Gmail that aren't also being sent to your iCloud account.
6. Click Continue and select 'Forward it to': youraccount@icloud.com
7. Tick anything else you might like, such as 'Mark as read'
8. Save
9. Test it out!