Theory:

Symploce is a type of repetition, which is a combination of anaphora and epiphora.
Anaphora has the repetition at the beginning of a line, stanza, or a paragraph. On the other hand, epiphora has the repetition at the end.
 
However, in the case of symploce, the repetition appears both in the beginning and at the end of a sentence, stanza, paragraph, poem, or prose.
Pronunciation Guide
Symploce: Symp (as in "simple")-- lo (as in "low")-- ce ( as in "see")
Let us take a look at the following example. It is by far one of the most famous examples of symploce seen in the English literature.
Example:
O Captain! my Captain! our fearful trip is done,
The ship has weather’d every rack, the prize we sought is won,
The port is near, the bells I hear, the people all exulting,
While follow eyes the steady keel, the vessel grim and daring;
But O heart! heart! heart!
O the bleeding drops of red,
Where on the deck my Captain lies,
Fallen cold and dead.
 
O Captain! my Captain! rise up and hear the bells;
Rise up—for you the flag is flung—for you the bugle trills,
For you bouquets and ribbon’d wreaths—for you the shores a-crowding,
For you they call, the swaying mass, their eager faces turning;
Here Captain! dear father!
This arm beneath your head!
It is some dream that on the deck,
You’ve fallen cold and dead.     
                  
My Captain does not answer, his lips are pale and still,
My father does not feel my arm, he has no pulse nor will,
The ship is anchor’d safe and sound, its voyage closed and done,
From fearful trip the victor ship comes in with object won;
Exult O shores, and ring O bells!
But I with mournful tread,
Walk the deck my Captain lies,
Fallen cold and dead.
In the above poem "O Captain! My Captain" by Walt Whitman, we could find two kinds of repetitions. A set of repetitions appear at the beginning of the stanzas. So, the phrase "O Captain! My Captain" is the anaphora. The line "fallen cold and dead" which appears at the end of each stanza is the epiphora.  "O Captain! My Captain" and "fallen cold and dead" together will become an example of symploce.
 
William Shakespeare had also used this rhetorical device in poems. Let us take the following poem  "Fear No More"  as another example to understand the concept.
Example:
Fear no more the heat o’ the sun,
Nor the furious winter’s rages;
Thou thy worldly task hast done,
Home art gone, and ta’en thy wages:
Golden lads and girls all must,
As chimney-sweepers, come to dust.

Fear no more the frown o’ the great,
Thou art past the tyrant’s stroke;
Care no more to clothe and eat;
To thee the reed is as the oak:
The sceptre, learning, physic, must
All follow this, and come to dust.

Fear no more the lightning-flash,
Nor the all-dreaded thunder-stone;
Fear not slander, censure rash;
Thou hast finished joy and moan:
All lovers young, all lovers must
Consign to thee, and come to dust.
The phrases "Fear no more" and "come to dust" together will be known as symploce.
Hence, if a text has both anaphora and epiphora, then the repetition is known as symploce.
Symploce used in the poem "Macavity: The Mystery Cat":
Macavity’s a Mystery Cat: he’s called the Hidden Paw —
For he’s the master criminal who can defy the Law.
He’s the bafflement of Scotland Yard, the Flying Squad’s despair:
For when they reach the scene of crime — Macavity’s not there!
 
Macavity, Macavity, there’s no one like Macavity,
He’s broken every human law, he breaks the law of gravity.
His powers of levitation would make a fakir stare,
And when you reach the scene of crime — Macavity’s not there!
You may seek him in the basement, you may look up in the air —
But I tell you once and once again, Macavity’s not there!
 
Macavity’s a ginger cat, he’s very tall and thin;
You would know him if you saw him, for his eyes are sunken in.
His brow is deeply lined with thought, his head is highly domed;
His coat is dusty from neglect, his whiskers are uncombed.
He sways his head from side to side, with movements like a snake;
And when you think he’s half asleep, he’s always wide awake.
 
Macavity, Macavity, there’s no one like Macavity,
For he’s a fiend in feline shape, a monster of depravity.
You may meet him in a by-street, you may see him in the square —
But when a crime’s discovered, then Macavity’s not there!
The line "Macavity’s not there!" is epiphora, and whereas the line "Macavity, Macavity, there’s no one like Macavity" is anaphora. Hence, when put together, they are known as symploce.
    Reference:
  • https://www.poetryfoundation.org/poems/45474/o-captain-my-captain
  • National Council of Educational Research and Training (2006). Beehive. Fear No More-William Shakespeare (pp. 137). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.