mention that imported messages are not deduplicated

so importing twice can result in duplicates.

related to issue #180

doc.go

@@ -695,6 +695,9 @@ recipients to be accepted, unless other reputation signals prevent that.
 Users can also import mailboxes/messages through the account web page by
 uploading a zip or tgz file with mbox and/or maildirs.
 
+Messages are imported even if already present. Importing messages twice will
+result in duplicate messages.
+
 Mailbox flags, like "seen", "answered", will be imported. An optional
 dovecot-keywords file can specify additional flags, like Forwarded/Junk/NotJunk.
 
@@ -722,6 +725,9 @@ recipients to be accepted, unless other reputation signals prevent that.
 Users can also import mailboxes/messages through the account web page by
 uploading a zip or tgz file with mbox and/or maildirs.
 
+Messages are imported even if already present. Importing messages twice will
+result in duplicate messages.
+
 	usage: mox import mbox accountname mailboxname mbox
 
 # mox export maildir

import.go

@@ -42,6 +42,9 @@ recipients to be accepted, unless other reputation signals prevent that.
 
 Users can also import mailboxes/messages through the account web page by
 uploading a zip or tgz file with mbox and/or maildirs.
+
+Messages are imported even if already present. Importing messages twice will
+result in duplicate messages.
 `
 
 func cmdImportMaildir(c *cmd) {

webaccount/account.js

@@ -1586,7 +1586,7 @@ const index = async () => {
 		}
 	}, importFieldset = dom.fieldset(dom.div(style({ marginBottom: '1ex' }), dom.label(dom.div(style({ marginBottom: '.5ex' }), 'File'), dom.input(attr.type('file'), attr.required(''), attr.name('file'), function focus() {
 		mailboxFileHint.style.display = '';
-	})), mailboxFileHint = dom.p(style({ display: 'none', fontStyle: 'italic', marginTop: '.5ex' }), 'This file must either be a zip file or a gzipped tar file with mbox and/or maildir mailboxes. For maildirs, an optional file "dovecot-keywords" is read additional keywords, like Forwarded/Junk/NotJunk. If an imported mailbox already exists by name, messages are added to the existing mailbox. If a mailbox does not yet exist it will be created.')), dom.div(style({ marginBottom: '1ex' }), dom.label(dom.div(style({ marginBottom: '.5ex' }), 'Skip mailbox prefix (optional)'), dom.input(attr.name('skipMailboxPrefix'), function focus() {
+	})), mailboxFileHint = dom.p(style({ display: 'none', fontStyle: 'italic', marginTop: '.5ex' }), 'This file must either be a zip file or a gzipped tar file with mbox and/or maildir mailboxes. For maildirs, an optional file "dovecot-keywords" is read additional keywords, like Forwarded/Junk/NotJunk. If an imported mailbox already exists by name, messages are added to the existing mailbox. If a mailbox does not yet exist it will be created. Messages are not deduplicated, importing them twice will result in duplicates.')), dom.div(style({ marginBottom: '1ex' }), dom.label(dom.div(style({ marginBottom: '.5ex' }), 'Skip mailbox prefix (optional)'), dom.input(attr.name('skipMailboxPrefix'), function focus() {
 		mailboxPrefixHint.style.display = '';
 	})), mailboxPrefixHint = dom.p(style({ display: 'none', fontStyle: 'italic', marginTop: '.5ex' }), 'If set, any mbox/maildir path with this prefix will have it stripped before importing. For example, if all mailboxes are in a directory "Takeout", specify that path in the field above so mailboxes like "Takeout/Inbox.mbox" are imported into a mailbox called "Inbox" instead of "Takeout/Inbox".')), dom.div(dom.submitbutton('Upload and import'), dom.p(style({ fontStyle: 'italic', marginTop: '.5ex' }), 'The file is uploaded first, then its messages are imported, finally messages are matched for threading. Importing is done in a transaction, you can abort the entire import before it is finished.')))), importAbortBox = dom.div(), // Outside fieldset because it gets disabled, above progress because may be scrolling it down quickly with problems.
 	importProgress = dom.div(style({ display: 'none' })), dom.br(), footer);

webaccount/account.ts

@@ -1362,7 +1362,7 @@ const index = async () => {
 							mailboxFileHint.style.display = ''
 						}),
 					),
-					mailboxFileHint=dom.p(style({display: 'none', fontStyle: 'italic', marginTop: '.5ex'}), 'This file must either be a zip file or a gzipped tar file with mbox and/or maildir mailboxes. For maildirs, an optional file "dovecot-keywords" is read additional keywords, like Forwarded/Junk/NotJunk. If an imported mailbox already exists by name, messages are added to the existing mailbox. If a mailbox does not yet exist it will be created.'),
+					mailboxFileHint=dom.p(style({display: 'none', fontStyle: 'italic', marginTop: '.5ex'}), 'This file must either be a zip file or a gzipped tar file with mbox and/or maildir mailboxes. For maildirs, an optional file "dovecot-keywords" is read additional keywords, like Forwarded/Junk/NotJunk. If an imported mailbox already exists by name, messages are added to the existing mailbox. If a mailbox does not yet exist it will be created. Messages are not deduplicated, importing them twice will result in duplicates.'),
 				),
 				dom.div(
 					style({marginBottom: '1ex'}),