Don’t invite a friend to eat by you when you know he won’t eat. Don’t offer him a gift when you know he won’t accept it. And similarly anything like it. For this is “one thing in the mouth, and another in the heart” [i.e. duplicity]. If he shows his friend that he honors him, but the intent is not whole [not genuine or not as great as the display], it is prohibited. Rather, at all times he should have his mouth and his heart in consonance, and he should conduct himself with lips of truth, a correct will/spirit, and a pure heart.
The short coda to the Amidah, “Elokai, Netzor“, written by Mar berei deRavina as a personal request following his formalized prayers (Berakhos 17b), begins with “אלהי, נצור לשוני מרע ושפתותי מדבר מרמה — my G-d, stop my tongue from evil, and my lips from speaking mduplicity.” This an adaptation of Tehillim 34:14 (“נְצֹ֣ר לְשֽׁוֹנְךָ֣ מֵרָ֑ע וּ֝שְׂפָתֶ֗יךָ מִדַּבֵּ֥ר מִרְמָֽה׃”), but where David advises the children to strive for this, Mar berei deRavina asks for G-d’s help to be able to accomplish it in his own life.
The Vilna Gaon notes that the difference between the two halves of this line (and the two haves of the next two) is whether we allow the evil to be expressed, or keep it contained within. In the first case, our language is evil. In the second, our lips, the border of our mouths, keeps the hatred within and we falsely speak words of friendship. This compounds the problem of geneivas da’as, lying, with that of violating the verse “lo sisna es achikha bilvavekha — do not hate your brother in your heart”, rather than airing grievances and working them out. (I discussed this idea at greater length last year.)