A single hostname can have 2 lines, one for IPv4 and one for IPv6. The man page should be updated to reflect that.
I think this report has been addressed by the following patch from Thomas Bartelsmeier, so I'm closing this.
diff --git a/man5/hosts.5 b/man5/hosts.5
index cb055f5f6..1230f9303 100644
@@ -42,6 +42,7 @@ line should be present with the following information:
IP_address canonical_hostname [aliases...]
+The IP address can conform to either IPv4 or IPv6.
Fields of the entry are separated by any number of blanks and/or
Text from a "#" character until the end of the line is
@@ -53,6 +54,8 @@ alphabetic character and end with an alphanumeric character.
Optional aliases provide for name changes, alternate spellings,
shorter hostnames, or generic hostnames (for example,
.IR localhost ).
+If required, a host may have two separate entries in this file;
+one for each version of the Internet Protocol (IPv4 and IPv6).
The Berkeley Internet Name Domain (BIND) Server implements the
Internet name server for UNIX systems.